The investigation by Department of Justice Inspector General Michael Horowitz, first announced in March 2018, initially sought to examine the circumstances surrounding the FBI's surveillance of a then-Trump campaign adviser Carter Page -- who had lived and worked in Russia.
In October 2016, the FBI filed a surveillance warrant with the Foreign Intelligence Surveillance Court against Page, and provided a series of materials to the court that agents said supported their belief that Page was "the subject of targeted recruitment by the Russian government," according to a since-declassified copy of the application.
A since-declassified copy of the warrant indicated its monitoring of Page's communications was part of a broader investigation of Russia's alleged efforts to, "undermine and influence the outcome of the 2016 U.S. presidential election in violation of U.S. criminal law."
The FISA application was renewed a total of three times, each time with the approval of Republican-appointed judges, before it was allowed to expire in September 2017.
While Horowitz's investigation was initially believed to be narrowly targeted in its review of the Page FISA application, it expanded over the past year and a half into a broader review of the conduct of senior FBI and Department of Justice officials in the early days of the Russia investigation. Horowitz's original mandate noted that, "if circumstances warrant, [he would] consider including other issues that may arise during the course of the review."
According to sources familiar with the review, Horowitz looked into whether FBI officials Peter Strzok and Lisa Page, who privately exchanged anti-Trump text messages while working on the Russia probe, were guided by politics in their official actions. He also probed whether senior Justice Department official Bruce Ohr improperly tried to influence the probe by sharing Steele's information with the FBI, even though the agency had already received much of Steele's information from elsewhere.
Attorney General William Barr has additionally announced a separate investigation led by U.S. attorney John Durham looking at the conduct of intelligence agency officials who initiated the counterintelligence investigation of the Trump campaign.
The scrutiny has provided political fodder for many Republican lawmakers who have argued the investigation into the Trump campaign was tainted by political bias. Trump has taken that claim even further, repeatedly suggesting without evidence that Horowitz's review will be explosive and validate his unfounded claims that senior FBI and DOJ officials were looking to undermine his campaign and eventually sabotage his presidency.
"I predict you will see things that you don't even believe," Trump said in October. "The level of corruption -- whether it's (former FBI Director James) Comey; whether it's (former FBI agent Peter) Strzok and his lover, (former FBI lawyer Lisa) Page; whether it's so many other people -- (former FBI deputy director Andrew) McCabe; whether it's President Obama himself."
Horowitz delivered a draft version of his report the Department of Justice in mid-September, and ABC News confirmed last month that it includes at least one criminal referral. The referral alleges former FBI lawyer Kevin Clinesmith altered a document related to Page's FISA application, though sources emphasized that the alleged alteration did not have had any material impact on the overall appropriateness of the FISA warrant.
Sources say Horowitz's draft report is hundreds of pages in length and includes significant criticism of the conduct of FBI and DOJ officials involved in the investigation, however Horowitz is not expected to argue that the decision to launch the investigation was without proper cause.
Horowitz will testify regarding his findings before the Senate Judiciary Committee Wednesday, and committee chairman Sen. Lindsey Graham has said he expects the inspector general will personally make "recommendations as to how to make our judicial and investigative systems better."
Republicans are expected to seize on the report as validating some of their long-expressed concerns about the investigation amid the backdrop of the ongoing impeachment inquiry over the alleged pressure campaign by President Trump to have Ukraine announce an investigation into 2020 presidential political rival and former Vice President Joe Biden..
In Tuesday's impeachment hearing in the House Judiciary committee, Republican Rep. Matt Gaetz suggested, without providing evidence, that Horowitz's report would be so damning that it would shift the focus of impeachment to former President Barack Obama.
"If wiretapping political opponents is a political offense, I look forward to reading that inspector general's report because maybe it's a different president we should be impeaching," Gaetz said.
With Durham's investigation still ongoing, it's unlikely Horowitz's report and testimony will provide any definitive closure for critics of the investigation like Gaetz and President Trump.
"As you know, the big one that's going to come out is the Durham report," Trump told reporters last Wednesday. "And I don't know Mr. Durham. I've never spoken to him. But he's one of the most respected law enforcement or U.S. attorneys anywhere in the country. He's a tough guy."
It's unclear whether Durham has uncovered significant information beyond what Horowitz found, though in late October ABC News confirmed that his investigation had formally turned into a criminal probe, at least in part due to the criminal referral of Clinesmith, sources say.
The move provides Durham powers that Horowitz lacked, such as convening a grand jury and issuing subpoenas for witnesses and documents.
But it has also been a source of concern among Democrats who have cast Barr as an unreliable and partisan attorney general, who has made clear through his public statements of his skepticism to how the investigation was handled.
"I assumed I'd get answers when I went in and I have not gotten answers that are, well, satisfactory," Barr said in a May interview. "In fact probably have more questions, and that some of the facts that- that I've learned don't hang together with the official explanations of what happened."
A source familiar with the process said that Barr is not currently expected to offer a formal response to the Horowitz report upon its transmission to Congress that the DOJ at times includes with IG reports critical of the department.
ABC News' Mike Levine contributed reporting to this article.