Rod Rosenstein, Andrew McCabe emerge as key players in firing of FBI Director James Comey

— -- Amid the controversy over the unexpected firing of FBI Director James Comey, two officials in Washington -- U.S. Deputy Attorney General Rod Rosenstein and Acting FBI Director Andrew McCabe -- have emerged as key players in the move and could be critical in what happens next.

Here’s an explainer on their involvement:

US Deputy Attorney General Rod Rosenstein

Multiple top White House officials have pointed the finger at little-known U.S. Deputy Attorney General Rod Rosenstein as the person who apparently made the case that Comey should be let go.

Speaking to reporters on Capitol Hill this morning, Vice President Mike Pence gave his first reaction to Comey’s dismissal and, like several administration officials before him, insisted that the initial recommendation was unexpected and came from Rosenstein.

“[Rosenstein] brought the recommendation to the president,” Pence told reporters. “The attorney general concurred with that recommendation and I, personally, am grateful that we have a president who is willing to provide the kind of decisive and strong leadership to take the recommendation of the deputy attorney general and the attorney general and to remove the FBI director who had lost the confidence in the American people.”

Pence denied that President Donald Trump’s decision to accept the recommendation had anything to do with the FBI’s ongoing investigation into Russian interference in the 2016 U.S. election. Rosenstein is also tasked with overseeing the investigation, after Sessions recused himself in March.

Comey was fired from his position because of his handling of the agency’s probe into Hillary Clinton's use of a private email server during her time as secretary of state, according to the White House.

In a letter made public by the White House, Trump wrote directly to Comey informing him of the move, citing the recommendations of Rosenstein and U.S. Attorney General Jeff Sessions.

Rosenstein’s recommendation letter to the president was also released, wherein the deputy attorney general credited Comey with being "an articulate and persuasive public speaker about leadership," though going on to note that he "cannot defend the Director's handling of the conclusion of the investigation of Secretary Clinton's emails, and I do not understand his refusal to accept the nearly universal judgment that he was mistaken."

"Almost everyone agrees that the Director made serious mistakes; it is one the few issues that unites people of diverse perspectives," Rosenstein wrote.

The letter goes on to allege that Comey was wrong to later "hold press conferences to release derogatory information about the subject of a declined criminal investigation."

White House press secretary Sean Spicer told reporters Tuesday night that the recommendations for the FBI director's termination arrived on the president's desk unexpectedly, and Rosenstein was the point man.

When asked who directed the deputy attorney general to conduct a review of Comey, Spicer replied, "That was all him," before adding, "I should say that, no one from the White House; that was DOJ decision."

It may come as a surprise to some that Rosenstein’s name is at the center of one of Trump’s most alarming decisions thus far.

When Trump nominated Rosenstein to be deputy attorney general, law professor Jonathan H. Adler wrote on the law blog Volokh Conspiracy that the then-U.S. attorney for Maryland was a “reassuring choice” and “one that should be completely free of controversy.”

Thiru Vignarajah, the former deputy state attorney general of Maryland who once worked for Rosenstein, wrote in an April op-ed for Vox that Rosenstein “enjoyed rare bipartisan support and was the only United States attorney in the country appointed by President George W. Bush who remained in the role at the end of the Obama administration.”

ABC News has learned that Rosenstein, along with Sessions, is now interviewing candidates to take over the FBI as interim director. They are focusing their search mostly on senior officials in the FBI and the U.S. Department of Justice, including the heads of FBI field offices around the country. A candidate is expected to be chosen and announced in the coming hours or days, while the search continues for a permanent replacement.

Rosenstein and Sessions today are interviewing FBI Executive Assistant Director Paul Abbate, who oversees all FBI criminal and cyber investigations worldwide, National Counterintelligence Executive William Evanina of the Office of the Director of National Intelligence, Special Agent-in-Charge of the FBI’s Richmond Division Adam Lee and Special Agent in Charge of the FBI’s Chicago Division Michael Anderson, according to a Justice Department official.

Acting FBI Director Andrew McCabe

FBI Acting Director Andrew McCabe is also under consideration to fill Comey’s role. Rosenstein and Sessions met with McCabe on Tuesday, the Justice Department official told ABC News.

In an interview with ABC News Tuesday night, White House press secretary Sean Spicer said Trump met with McCabe in the Oval Office but wouldn’t comment further on the meeting.

In the hours after Comey's dismissal Tuesday, McCabe held a conference call with the heads of FBI field offices across the country, telling colleagues to carry on with their jobs, sources told ABC News. One source described McCabe's bottom line as, "business as usual."

Fearing interference from the White House, Democratic members of the House of Representatives penned a letter to McCabe and Rosenstein Tuesday, urging them to “protect the integrity of your investigations into Russian efforts to influence our recent election and related matters.”

They also asked that “associated matters be preserved and placed off-limits to any and all White House officials.”

McCabe joined the FBI 20 years ago, starting his career at the New York field office and focusing on organized crime. He was appointed deputy director by Comey last year. Now, he’s temporarily leading the bureau.

McCabe is not entirely new to controversy. On Jan. 12, the U.S. Department of Justice’s Office of the Inspector General announced that it was looking into allegations that McCabe should have been recused from participating in “certain investigative matters.”

On March 28, Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, wrote a letter to Comey, calling on the FBI director to detail McCabe’s involvement in the agency’s ongoing Russia investigating.

Grassley expressed concerns that the FBI’s then-deputy director was under review for “political conflicts in the Clinton email matter” and referenced media reports that his wife received nearly $700,000 in campaign contributions from longtime Clinton associates when she was running for Virginia state senate.

“These circumstances undermine public confidence in the FBI’s impartiality … FBI’s senior leadership should never have allowed that appearance of a conflict to undermine the Bureau’s important work. … If Mr. McCabe failed to avoid the appearance of a partisan conflict of interest in favor of Mrs. Clinton during the presidential election, then any participation in this inquiry creates the exact same appearance of a partisan conflict of interest against Mr. Trump,” Grassley wrote in the letter.

Grassley’s statement on Tuesday regarding Comey’s firing didn’t mention his concerns about McCabe.

ABC News' Riley Beggin, Benjamin Bell, Jack Date, Justin Fishel, Jonathan Karl, Meghan Keneally, Mike Levine, Alexander Mallin, Benjamin Siegel, and Alex Stone contributed to this report.