Trump has since acknowledged discussing Biden on the call, but denies any wrongdoing.
Much is still unclear about what led to the whistleblower report and there's much that's still unknown, but here's some context and a timeline of important developments that occurred around the complaint and in its aftermath:
June 13, 2019: Trump tells ABC News' Chief Anchor George Stephanopoulos that he would consider accepting damaging information about a political rival from a foreign country without alerting the FBI first
Asked by ABC News Chief Anchor George Stephanopoulos, in an Oval Office interview, whether his campaign would accept damaging information on opponents from foreigners -- such as China or Russia -- or hand it over to the FBI, Trump said, "I think maybe you do both."
"I think you might want to listen, there isn't anything wrong with listening," Trump continued. "If somebody called from a country, Norway, [and said] 'we have information on your opponent' -- oh, I think I'd want to hear it."
June 18, 2019: The Department of Defense announces $250 million in aid for Ukraine
In a press release, the Department of Defense announced plans to provide equipment to support military operations in Ukraine. According to the press release, the funds were being given in part to purchase "sniper rifles, rocket-propelled grenade launchers, and counter-artillery radars" among other things. At some point, the aid is put on hold.
Mid-July, 2019: Trump instructs Acting Chief of Staff, Mick Mulvaney to put a hold on disbursement of aid funds to Ukraine, according to multiple senior administration official
Officials at the Office of Management and Budget relayed Trump’s order to the State Department and the Pentagon during an interagency meeting in mid-July, according to officials who spoke on the condition of anonymity to discuss internal deliberations. The Office of Management and Budget officials explained that the president had “concerns” and wanted to analyze whether the money needed to be spent.
Administration officials were instructed to tell lawmakers that the delays were part of an “interagency process” but to give them no additional information.
July 24, 2019: Former special counsel Robert Mueller testifies on Capitol Hill
Mueller testified before two House committees about his investigation into Russian interference in the 2016 election and potential obstruction of justice by the president.
July 25, 2019: Trump partakes in the now controversial phone call with the new president of Ukraine, Volodymyr Zelenskiy
Trump took part in what he has referred to as a "routine" phone call with the newly elected Ukranian leader. The substance of what he said in this phone call reportedly prompted the whistleblower complaint.
July 28, 2019: Trump announces that Dan Coats will be leaving his role as director of national intelligence
Three days after the call with Ukraine, Trump tweets that Coats, who had been serving as the director of national intelligence since March 2017, would be stepping down. Trump also tweeted his intent to nominate Rep. John Ratcliffe to fill the role.
Aug. 8, 2019: Trump changes course on DNI nomination, Coats' top deputy resigns
After a failed attempt to rally support for Ratcliffe, Trump announces that he will instead name Joseph Maguire to fill in for Coats. Many lawmakers on both sides of the aisle -- as well as some members of the intelligence community -- had hoped that Trump would name Sue Gordon, Coats' second in command, to fill the role, but Trump announced in a tweet on Aug. 8 that Gordon would also be departing.
Aug. 12, 2019: The whistleblower files a complaint with the intelligence community inspector general
An unidentified member of the intelligence community filed a complaint with Michael Atkinson, the inspector general for the intelligence community. In the complaint, the whistleblower outlined several instances of concern related to the president's communications with a foreign leader. The complaint has not been made publicly available, but the inspector general revealed the nature of the complaint in a letter, and sources have told ABC News other details of the complaint.
The whistleblower's filing would not be reported publicly for almost a month.
Aug. 16, 2019: Joseph Maguire assumes role of acting director of national intelligence
The former director of the National Counterterrorism Center is sworn in to replace Coats on an "acting" basis.
Aug. 26, 2019: Atkinson passes the whistleblower complaint to Maguire
After two weeks of independent investigation, Atkinson determined the whistleblower complaint viable and of "urgent concern." He forwarded the complaint to Maguire.
Sept. 2, 2019: Deadline passes for Maguire to alert Congress to the whistleblower report
Upon receipt of a report from the inspector general, the law gives the director of national intelligence one week to determine whether the concern rises to the level of requiring a report to the House and Senate Intelligence committees. Maguire does not alert Congress to the whistleblower report by this deadline.
Sept. 9, 2019: Atkinson expresses his concerns to House Intelligence Committee Chairman Adam Schiff and ranking member Devin Nunes in a letter
After Maguire did not disclose the information to Congress, Atkinson did. In a letter to the top Democrat and Republican on the House Intelligence committee, Atkinson wrote that he had "received a disclosure form an individual regarding an alleged 'urgent concern.'"
Sept. 10, 2019: Schiff pens a formal request for the full whistleblower complaint
Schiff responded to Atkinson's letter by issuing a request for the full whistleblower complaint, the inspector general's formal determination on the complaint, and any documents related to the director of national intelligence's involvement in the matter.
Sept. 11, 2019: The State Department gives Congress notice it wants to send an additional $141 million in aid to Ukraine
These funds were announced in addition to the $250 million previously allotted.
Sept. 11, 2019: National security adviser John Bolton leaves the Trump administration
Trump announced the departure of his top national security official via Twitter.
"I informed John Bolton last night that his services are no longer needed at the White House. I disagree strongly with many of his suggestions, as did other and the administration, and therefore… I asked John for his resignation, which was given to me this morning," Trump tweeted.
Bolton tweeted back that he had offered to resign the night before.
Sept. 11, 2019: The administration notifies Congress of plans to move forward with disbursing aid money for Ukraine
The administration agrees to release previously frozen aid money to Ukraine.
Sept. 13, 2019: Maguire denies Schiff's request for a copy of the complaint and related documents, Schiff issues a subpoena
Maguire responds to Schiff's letter in a letter of his own, stating his reason for denying Schiff's request for an unredacted copy of the complaint and related documents. According to Maguire, the information in the complaint involves "confidentially and potentially privileged communications by persons outside the Intelligence community."
Schiff responded to the letter by issuing a subpoena for the previously requested documents.
Sept. 18, 2019: News that the whistleblower complaint was about Trump breaks in the Washington Post
The Washington Post reports that Trump has been tied to a whistleblower complaint concerning a phone call with a "foreign leader." Later reports confirm that the complaint was about his July 25 call with Zelenskiy.
Sept. 19, 2019: Atkinson meets with the House Intelligence Committee behind closed doors
Schiff, speaking to reporters after the hearing, said that his committee does not have the complaint and does not know whether reports about it are accurate. He also said he believed there were efforts to keep details of the report out of the hands of Congress.
"I believe that there is an effort to prevent this information getting to Congress, and if the assertion is accurate that the Department of Justice has made and the DNI has affirmed, that this involves a potentially privileged communication, then at one level or another, likely involves either president or people around him," he said.
Sept. 20, 2019: Trump tries to redirect focus to Biden
Trump tells a group of reporters that his communications with foreign officials are "always appropriate." He also pointed questions back to Biden.
"It doesn't matter what I discussed, but I'll tell you this, somebody ought to look into Joe Biden's statement," Trump said.
Sept. 20, 2019: The Wall Street Journal reports that Trump urged Zelenskiy to work on an investigation into Biden's son about eight different times
Sept. 21, 2019: Ukraine foreign minister says that Trump did not put pressure on Zelenskiy during the July call
Ukraine foreign minister Vadym Prystaiko told a Ukrainian media outlet that Trump did not put pressure on the Ukraine president during the phone call.
"I know what the conversation was about and I think there was no pressure," Prystaiko said. "This conversation was long, friendly and it touched on many questions, sometimes requiring serious answers."
Sept. 22, 2019: Trump acknowledges discussing Biden with Ukraine's president
Trump defended his July 25 phone call with Zelenskiy while speaking with reporters before departing for a trip.
"The conversation I had was largely congratulatory, with largely corruption, all of the corruption taking place and largely the fact that we don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine," Trump said.
Sept. 22, 2019: Schiff suggests that the contents of the report could move the needle closer to impeachment
Schiff appeared on CNN saying impeachment might be the "only remedy" if reports of Trump withholding military aid were true.
"This seems different in kind and we may very well have crossed the Rubicon here," Schiff said.
Sept. 22, 2019: House Speaker Nancy Pelosi releases a letter calling for the administration to allow Congress access to the whistleblower complaint
In her letter, Pelosi wrote that the blocking of the complaint marked a "grave new chapter of lawlessness which will take us into a whole new stage of investigation."
Sept. 24, 2019: Pelosi announces that the House will move forward with a formal impeachment inquiry
"The actions of the Trump presidency revealed the dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections," Pelosi said in her public remarks. ""Therefore, today I'm announcing the House of Representatives is moving forward with an official impeachment inquiry."
Sept. 25, 2019: The White House releases a rough transcript of Trump's phone call with Zelenskiy
The record reveals that Trump repeatedly encouraged Zelenskiy to work with Giuliani and Attorney General Bill Barr to investigate Biden and his son.
"Rudy very much knows what’s happening and he is a very capable guy," the transcript shows Trump saying. "If you could speak to him that would be great.”
The White House cautioned that the transcript was not verbatim.
Sept. 25, 2019: The Department of Justice's Office of Legal counsel declassifies and makes public a previously written legal opinion finding that Maguire acted properly in not forwarding the whistleblower complaint to Congress
The opinion determined that the whistleblower complaint was outside of the scope of the intelligence community and that Maguire acted properly by not reporting the complaint to Congress.
The opinion also confirmed that the information in the complaint was "received secondhand" by the whistleblower.
Sept. 26, 2019: Declassified copies of the whistle blower complaint and the inspector general's report to Maguire are made public
The whistleblower’s complaint recounted what led up to the phone call, during which Trump repeatedly pressured Zelenskiy to work with Giuliani and Attorney General William Barr to investigate Biden.
The complaint states, among other things, that Trump instructed Vice President Mike Pence to cancel his trip to Ukraine and made clear that he did not want to meet Zelenskiy "until he saw how Zelenskiy 'chose to act' in office".
Sept. 26, 2019: Maguire appears on Capitol Hill for public testimony before the House Intelligence Committee and for closed-door testimony before the Senate Intelligence Committee
During the open House hearing, Maguire said he believed the matter was "totally unprecedented."
He also said he believed the whistleblower acted in good faith.
"I think the whistleblower did the right thing," Maguire said.
Sept. 27, 2019: House Democrats issue subpoenas for Secretary of State Mike Pompeo and five other State Department officials
The subpoenas are for documents related to the State Department's role in facilitating meetings between Giuliani and Zelenskiy, including communications with Giuliani and records around Trump's phone call with Ukrainian President Zelenskiy in July.
Sept. 27, 2019: Kurt Volker resigns as special envoy to Ukraine
News of Volker's resignation was first reported by The State Press, the student newspaper at Arizona State University.
Sept. 27, 2019: It is first reported that the Trump administration used a separate system to store the record of his call with Zelenskiy
When the rough transcripts of two phone calls President Donald Trump had with foreign leaders leaked in the early days of his presidency, the procedure to store those logs changed, multiple sources familiar with the process told ABC News. Many calls between the president and world leaders started being stored in a secure server to avoid leaks. The sources who talked to ABC News did caution that it's unclear if the call records being stored were done so for national security or for political concerns.
Sept. 30, 2019: Democratic chairmen subpoena Giuliani for documents related to their impeachment probe
”I have received a Committee subpoena from 3 Commitees of the House. It raises substantial constitutional and legal issues as well as attorney-client and other privileges. These and other issues must all be considered before a proper decision can be made,” Giuliani said in a statement the day the subpoena was issued.
Oct. 1, 2019: Pompeo sends a letter to the leaders of three House committees, stating his intention to block State Department witnesses from appearing for testimony. Democrats respond to the letter threatening to fine Pompeo or withhold his salary
In the fierce letter, released on Twitter and addressed to House Foreign Affairs Committee chairman Eliot Engel, Pompeo blasted the depositions in the impeachment probe as rushed and potentially in violation of executive privilege, accused committee staff of not following protocol, and appeared to say the officials will not show up.
"Based on the profound procedural and legal deficiencies noted above, the Committee's request dates for depositions are not feasible," Pompeo concluded his letter.
In their response letter, Chairmen Schiff, Engel and Elijah Cummings write, "Secretary Pompeo now appears to have an obvious conflict of interest.... He should not be making any decisions regarding witness testimony or document production in order to protect himself or the President." They threaten to fine Pompeo or withhold his salary if he does not comply.
Oct. 2, 2019: State Department Inspector General Steve Linick meets with House and Senate committee staffers for an "urgent" briefing
Democratic Rep. Jamie Raskin said at the conclusion of the meeting that the documents provided by Linick appeared to be full of "conspiracy theories"
A separate Democratic source told ABC News based on the information they were presented, the documents appeared to be sent from the White House and to Pompeo and the State Department, and that they contained a bunch of Trump hotel folders with notes from interviews that took place at the offices of President Donald Trump's personal lawyer, Rudy Giuliani, with various Ukrainians.
Oct. 3, 2019: Volker appears on Capitol Hill for closed-door deposition
As part of his deposition, Volker disclosed text messages he exchanged with other American diplomats.
In the disclosed messages, Volker is communicating with Gordon Sondland, the U.S. ambassador to the European Union, and Bill Taylor, the top U.S. diplomat to Ukraine.
During their exchanges, Taylor wrote, "I think it's crazy to withhold security assistance for help with a political campaign."
Sondland responds to Taylor, saying that charge is "incorrect."
"Bill, I believe you are incorrect about President Trump's intentions. The President has been crystal clear: no quid pro quo’s of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign," Sondland says.
Oct. 6, 2019: An attorney representing the original whistleblower tells ABC News that he is now representing a second whistlebower with first-hand knowledge of some of the information in the whistleblower complaint. The attorney said the second whistleblower has spoken to the inspector general
The existence of a second whistleblower -- particularly one who can speak directly about events involving the president related to conversations involving Ukraine -- could undercut Trump's repeated insistence that the original complaint, released on Sept. 26, was "totally inaccurate."
Oct. 8, 2019: The Trump administration bars Sondland from testifying before several House committees moments before his testimony is set to begin.
Schiff said the "failure to produce this witness" would be considered "strong evidence of obstruction" of the impeachment probe.
Hours later, Schiff, Engel and Cummings announced they would subpoena Sondland for both his testimony and documents.
Oct. 8, 2019: Republican Sen. Lindsey Graham announces he intends to invite Giuliani to testify before the Senate Judiciary Committee
In a series of tweets, Graham, the chairman of the Senate Judiciary Committee, said he intends to invite Giuliani to testify about what he called "corruption in Ukraine" and "other improprieties involving Ukraine."
Democratic Sen. Dianne Feinstein, the committee’s ranking member, said she, too, would like to hear from Giuliani if he testified under oath.
"Democratic members have plenty of questions for Mr. Giuliani and this would give us an opportunity to help separate fact from fiction for the American people,” Feinstein said in a statement.
The Senate committee has not formally requested testimony from Giuliani.
Oct. 8, 2019: The White House sends a letter to Pelosi stating that the executive branch "cannot be expected to participate" in the House impeachment inquiry
The letter called into question what the White House argued were "points of irregularity in the investigation," including what it said were the president's inability to cross-examine witnesses, call witness, receive access to evidence or to have counsel present. The letter also argued that the inquiry, which has not been formally voted on by the full House, "lacks any legitimate constitutional foundation, any pretense of fairness, or even the most elementary due process protections."
"This letter is manifestly wrong, and is simply another unlawful attempt to hide the facts of the Trump Administration's brazen efforts to pressure foreign powers to intervene in the 2020 elections," Pelosi responded in a statement."
In a ruling on a separate matter on Oct. 25, D.C. District Court Chief Judge Beryl Howell endorsed the legality and constitutionality of the Democrats' impeachment proceedings.
Oct. 9, 2019: It is announced that former South Carolina Rep. Trey Gowdy will join the president's legal team
“I have known Trey for years and worked with him when he served in Congress. His legal skills and his advocacy will serve the President well," said Jay Sekulow, one of the president's personal lawyers, said in a statement.
It was unclear what specific role Gowdy would play in the current White House strategy of not cooperating with the investigation.
In the days following, sources told ABC News that Gowdy, facing legal concerns about his transition from elected office to the president's legal team, would no longer be joining the effort.
Oct. 10, 2019: Two men with ties to Giuliani are indicted on charges of connection to an alleged scheme to circumvent federal laws against foreign campaign donations
The two men, Lev Parnas and Igor Fruman, reportedly assisted Giuliani in his efforts to investigate the Biden family.
They were arrested on the evening of Oct. 9 at Washington Dulles International Airport attempting to leave the country with one-way tickets to Vienna.
Parnas and Fruman made initial appearances in federal court in the Eastern District of Virginia. They face four counts, including conspiracy to commit campaign finance fraud, making false statements to the Federal Election Commission, and falsification of records in the Southern District of New York.
Oct. 11, 2019: Former Ukraine Ambassador Marie Yovanovitch appears on Capitol Hill for deposition
Yovanovitch was recalled from her post in Ukraine earlier this year. In her prepared statements, Yovanovitch testified that although she was informed of no wrongdoing, Trump directly pressured the State Department to remove her from her post.
Oct. 14, 2019: Fiona Hill, a former top national security official on Russia, meets with lawmakers behind closed doors
Though she agreed to appear to appear voluntarily, Hill's attorney tweeted that she had been subpoenaed by the committee shortly before her arrival.
Hill met with the committee for roughly 10 hours.
"She was a remarkably thorough and authoritative witness who had truly amazing powers of recall of particular events and particular meetings," Raskin told reporters after the meeting.
Oct. 15, 2019: Giuliani, Pence, the Office of Management and Budget and the Department of Defense fail to meet deadlines to turn over subpoenaed documents to Congress
The move dramatically escalates the standoff between the Democratic-led House and the White House, which also has told administration officials not to testify and has otherwise stonewalled Congress.
"If they enforce it, then we will see what happens," Giuliani said of the congressional subpoena issued to him in late September. Congress has so far not taken formal enforcement action for the failure of Giuliani and other officials to meet subpoena deadlines.
Oct. 16, 2019: Michael McKinley, the former senior adviser to Mike Pompeo, appears on Capitol Hill for deposition.
McKinley, who resigned from his position just a week earlier, told congressional lawmakers that he decided to leave the State Department over the treatment of Yovanovitch. He testified that he asked Pompeo to show support for Yovanovitch, but that the secretary of state didn't respond, according to the source.
Oct. 17, 2019: Energy Secretary Rick Perry tells Trump he plans to resign amid controversy surrounding his involvement in Ukraine policy
Trump told reporters that Perry, described by colleagues as one of the "three amigos" on U.S. policy on Ukraine, will step down at the end of the year.
Perry's resignation notice came amid questions about his role in a White House effort to pressure Ukraine into launching an investigation that some conservatives thought would help Trump's re-election campaign. He has not been accused of any wrongdoing, and has insisted that actions he took regarding Ukraine were intended to advance U.S. interests in the region -- namely addressing government corruption and encouraging American companies to do business there.
Oct. 17, 2019: Sondland testifies before the three committees leading impeachment investigations.
Sondland tells Congress that Trump directed him and others to work with Giuliani to push Ukraine to announce investigations, but that he was in the dark on the extent of Giuliani's efforts and the political motive behind it, according to his opening statement obtained by ABC News.
Oct. 17, 2019: In a press conference in the White House briefing room, acting White House chief of staff Mick Mulvaney acknowledges a quid pro quo effort on Ukraine, saying that Trump had ordered him to withhold military aid in part to pressure Ukraine to launch an investigation of Democrats
After acknowledging that an investigation into the 2016 election was a consideration in the release of Ukrainian aid money, Mulvaney was challenged by ABC News' Chief White House Correspondent Jonathan Karl, who told Mulvaney that his description of events constituted a quid pro quo.
"I have news for everybody: Get over it," Mulvaney responded. "There's going to be political influence in foreign policy."
After hours of backlash, Mulvaney attempted to clarify his comments in a statement released by the White House.
“Once again, the media has decided to misconstrue my comments to advance a biased and political witch hunt against President Trump. Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election," Mulvaney noted. "The president never told me to withhold any money until the Ukrainians did anything related to the server. The only reasons we were holding the money was because of concern about lack of support from other nations and concerns over corruption."
Oct. 22, 2019: The top U.S. diplomat in Ukraine, Bill Taylor, testifies to members of Congress that "it was becoming clear" to him that a meeting between Trump and the Ukrainian President "was condition on the investigation of Burisma and alleged Ukrainian interference in the 2016 elections"
Taylor's comments, made in his opening statements obtained by several media outlets, were the strongest statements made by a member of the State Department regarding the growing impeachment inquiry. His statements suggested that meetings with Trump, as well as military aid to Ukraine, were dependent upon Ukrainian cooperation in investigations into Joe Biden and Burisma, an energy company on whose board Joe Biden’s son Hunter served. Taylor testified that Giuliani had been leading efforts to get Ukraine to open these investigations.
"The push to make President Zelenskiky publicly commit to investigations of Burisma and alleged interference in the 2016 election showed how the official foreign policy of the United States was undercut by the irregular efforts led by Mr. Giulinai," Taylor said in his opening statement.
Oct. 23, 2019: The two Giuliani associates indicted on campaign finance violations plead not guilty in a Manhattan federal court
Parnas and Fruman were each released on a $1 million bond and subjected to home confinement in Florida with GPS monitoring.
A defense attorney for Parnas, Edward MacMahon Jr., said that Parnas had retained Giuliani for personal and business legal matters and “worked for Giuliani” in an unspecified capacity. MacMahon said that he could potentially raise executive privilege issues during discovery because of Giuliani’s role as President Trump’s attorney.
Oct. 23, 2019: About two dozen House Republicans storm the secured hearing room where depositions take place. The move delays the deposition of Laura Cooper, a Department of Defense official, for about 5 hours
The GOP group, led by Rep. Matt Gaetz, R-Fla., staged a news conference denouncing the Democrats' impeachment inquiry procedures and alleging that many Republican members could not access material from the ongoing depositions. After the news conference, the group entered the secure room known as a Secure Compartmented Information Facility, or SCIF, where Republicans and Democrats on relevant committees were preparing to question Cooper, a senior Pentagon official with jurisdiction over Russian and Ukrainian policy.
More than two dozen Republicans entered the room at once. Of the members that took part in the entry, about a dozen serve on committees that are conducting the inquiry and are permitted to enter the secured area to listen to testimony.
Some unauthorized members remained in the room for several hours. Cooper's deposition eventually proceeded.
Oct. 25, 2019: A judge rules that the Justice Department must turn Mueller evidence over to House Committee. In her ruling, Chief Judge Beryl Howell endorses the legality of the House's impeachment proceedings
Republicans have demanded Democrats hold a formal vote on the House floor to launch impeachment proceedings to legitimize the inquiry, clarify its scope and provide them with additional rights in the minority. The White House accused Democrats of launching an "illegitimate" impeachment effort, and vowed to stonewall Congress in the absence of a formal floor vote. Howell dismissed the argument, accusing Republicans of cherry-picking historical precedent and writing that "no governing law requires this test" to officially launch impeachment.
"While close scrutiny of the historical record undercuts that justification for the “House resolution” test proposed by Representative [Doug] Collins, the more significant flaw with this proposal is as follows: while this test may address political legitimacy concerns, which are best resolved in the political arena, no governing law requires this test—not the Constitution, not House Rules, and not Rule 6(e), and so imposing this test would be an impermissible intrusion on the House’s constitutional authority both to “determine the rules of its proceedings," Howell wrote.
Oct. 25, 2019: Former national security official Charles Kupperman files a civil case asking a DC federal judge to determine whether he should comply with the House's subpoena for testimony
In the filing, Kupperman’s attorney asks the judge to sort out "irreconcilable commands" coming from the House Judiciary committee and the White House.
"Dr. Kupperman filed an action today in the United States District Court for the District of Columbia requesting that the Court issue a Declaratory Judgment determining whether he should comply with the House’s subpoena or with the President’s assertion of immunity and instruction that he not appear and testify," Kupperman's attorney, Charles Cooper, said in a statement. "Dr. Kupperman takes no position on whether the command of the Legislative Branch or the command of the Executive Branch should prevail; he seeks only to carry out whichever constitutional obligation the Judicial Branch determines to be lawful and binding on him."
Oct. 26, 2019: Philip Reeker, the acting Assistant Secretary of European and Eurasian affairs and a former ambassador, appeared with his lawyer on Saturday for a nearly nine-hour, closed-door interview with the three House committees leading an impeachment inquiry
In his testimony, Reeker said he thought there was no credence to the attacks on the former U.S. ambassador to Ukraine, Marie Yovanovitch, a source with direct knowledge of his testimony told ABC News.
Oct. 28, 2019: The Justice Department appeals a ruling allowing Democrats to access Mueller's grand jury material and questioned the relevance of the material in the ongoing impeachment inquiry
In their filing, lawyers for the DOJ pointed to House Speaker Nancy Pelosi's comments that the impeachment probe would focus primarily on the whistleblower complaint and issues surrounding the Ukraine affair.
"Although the HJC [House Judiciary Committee] claims that it needs the information promptly because it continues to investigate matters connected to the Mueller Report, there appears little dispute that, for now, that investigation is secondary, and Congressman Schiff and the House Intelligence Committee—not the Judiciary Committee—is the lead committee heading the congressional investigation," DOJ lawyers wrote.
Oct. 28, 2019: Kupperman declines to appear on Capitol Hill for his testimony
His attorneys point to the recently filed lawsuit seeking clarity about whether he must appear and state that Kupperman will abide by the court's decision in that case.
Oct. 29, 2018: Lt. Col. Alexander Vindman, a White House national Security Council official who listened in on the July 25 phone call, appears for deposition by the House committees
He testified before committees in his now-available testimony he was so "concerned" by Trump's comments that he raised alarms with a White House lawyer.
He was the first current White House official to appear before House investigators.
Oct. 30, 2019: Two State Department officials: Catherine Croft, a Ukraine specialist, and Christopher Anderson, the former special adviser to Kurt Volker, are deposed by the House investigative committees.
In her opening remarks, Croft testified about her time working for Volker and spoke to her participation in a mid-July 2019 meeting during which "an OMB representative reported that Mulvaney had placed an informal hold on security assistance to Ukraine."
Anderson told Congress that he recalled being in a meeting with then-national security adviser John Bolton in which Bolton "cautioned that Mr. Giuliani was a key voice with the President on Ukraine which could be an obstacle to increased White House engagement.
Oct. 31, 2019: The House of Representatives passes a resolution to authorize the ongoing impeachment inquiry, including open hearings, marking a new step in the investigation
The mostly party-line vote was the first time members of the House have formally gone on record to support or object to the ongoing impeachment inquiry.
Oct. 31, 2019: Attorneys representing the president, the House of Representatives and Kupperman appear in federal court for the first hearing in Kupperman's case
The hearing was largely procedural, but when the Justice Department asked the court for more time to build their case, the federal judge overseeing the lawsuit fired back.
"When it's a matter of this consequence to this country you roll up your sleeves and get the job done," U.S. Judge Richard Leon said.
Oct 31. 2019: Tim Morrison, the senior director for Europe and Russion on the White House National Security council, testifies on Capitol Hill
Sources told ABC News that Morrison, a close adviser of Bolton, plans to leave his post.
Morrison, the second current White House official to face investigators, corroborated elements of earlier testimony from William Taylor, the top U.S. diplomat in Ukraine, about the efforts to pressure Ukraine to open political investigations in exchange for military aid and a White House meeting between Trump and Zelenskiy.
But while Taylor told investigators the conversation between Sondland and an adviser to Zelenskiy focused on a public commitment to investigate from Ukraine's leader, Morrison, who relayed the account to Taylor, told Congress it was a commitment from Ukraine's prosecutor general to investigate.
Oct. 31, 2019: Bolton is summoned for testimony by House investigators
Though House investigators made a formal request for Bolton's testimony, he was not issued a subpoena.
Nov. 4, 2019: Four White House officials who are slated for depositions all decline to appear
Despite the House issuance of subpoenas for administration officials John Eisenberg, legal adviser to the national Security Counsel, Robert Blair, senior adviser to Mulvaney, Michael Ellis, associate counsel to the president and deputy legal adviser to the National Security Counsel, and Brian McCormick, associate director of Natural Resources and Energy at the Office of Management and Budget, none of the men appear for depositions with the House committee.
Nov. 4, 2019: House Democrats begin releasing transcripts of closed-door depositions
Full transcripts of the depositions of McKinley and Yovanovitch are released for the public to view. These are the first two transcripts to be released. House Democrats have continued releasing transcripts in the days following.
Nov. 5, 2019: More White House officials decline to appear for testimony before the House committees
Wells Griffith, the director of international energy at the National Security Council, and Michael Duffey, the association director for national security at the Office of Management and Budget, defy subpoenas and do not appear for depositions.
Nov. 6, 2019: Schiff announces the first public impeachment hearings scheduled for the following week
The move marks a significant ramping up of the impeachment inquiry.
Schiff announces the committees intend to hear publicly from Taylor and Kent on Nov. 13 and from Yovanovitch on Nov. 15.
Nov. 6, 2019: State Department official David Hale appears for closed-door testimony on Capitol Hill. Three other administration officials are also scheduled to appear on this date but do not appear
State Department counselor, T. Urlich Brechbul, acting White House budget director, Russ Vought and Perry do not appear.
Nov. 6, 2019: The House withdraws its subpoena for Kupperman's testimony and moves to have his case dismissed as moot
"The subpoena at issue in this matter has been withdrawn and there is no current intention to reissue it. Therefore, this matter is moot and should be dismissed," attorneys for the House wrote.
Attorneys on behalf of Kupperman have signaled that they do not wish to have the case dismissed because they want a court to determine whether he can testify.
Nov. 7, 2019: Bolton does not appear for his requested testimony before the House committees. Jennifer Williams, a special adviser to Europe and Russia in the office of the vice president, does appear.
Williams, is on detail from the State Department and does not directly advise the vice president. She was subpoenaed by the House and her lawyer stated that Williams' testimony would reflect what had already been made public.
Nov. 8, 2019: Mulvaney is subpoenaed for testimony by the House. He does not appear and files a motion to join the ongoing Kupperman lawsuit
Mulvaney's attorney argued that his interests were represented by the Kupperman case.
"Mr. Mulvaney, like Mr. Kupperman, finds himself caught in that division, trapped between the comands of two of its co-equal branches -- with one of those branches threatening him with contempt."
Nov. 9 2019: House Republicans release their request for witnesses they'd like to see called for public testimony
The list includes a name for the unidentified whistleblower, the former vice president’s son Hunter, and three witnesses who had previously appeared for closed-door depositions: Volker, Hale and Morrison.
Schiff later stated he would not allow the whistleblower to be called as a witness, arguing that the whistleblower's identity should be protected, but he has not weighed in on whether the other requested witnesses would be given a public hearing.
Nov. 11, 2019: Mulvaney withdraws his motion to join Kupperman's lawsuit. The next morning, he states that he will not refile his own lawsuit
After the judge in the case held a telephone conference between Mulvaney's lawyers and lawyers representing Kupperman, the House of Representatives and the president, Mulvaney filed a notice that he would no longer seek to join Kupperman's case.
Though he initially stated his intention to file a separate related lawsuit, the following morning Mulvaney said he would no longer seek the court's guidance.
"Rather, he will rely on the direction of the President, as supported by an opinion of the Office of Legal counsel of the U.S. Department of Justice, in not appearing for the relevant deposition," the filing reads.