The 448-page report detailing special counsel Robert Mueller's two-year Trump-Russia investigation is public. Next up: Department of Justice Inspector General Michael Horowitz's report into how that investigation started.

That report, anticipated to be released after Congress returns from August recess, will come amid an ongoing contentious debate between Republicans who think the government unfairly targeted President Trump and Democrats who believe Trump deserved to be investigated for his alleged ties to Russia.

But what Horowitz's report, which is expected to delve into alleged abuse of the Foreign Intelligence Surveillance Act and the FBI's use of the salacious and unverified dossier compiled by British ex-spy Christopher Steele, contains has been held close.

Still, recent developments have offered a sneak preview of the cast of characters who could make appearances when Horowitz explains what his team uncovered.

Former FBI Director James Comey received good news a couple weeks ago when the DOJ decided not to charge him over leaking his memos, despite a criminal referral from Horowitz, who concluded Comey leaked classified information and showed a lack of candor. But the leak inquiry was just a narrow slice of the broader investigation Horowitz launched in March of last year.

Horowitz is likely scrutinizing every aspect of the FISA approval process, and the head of the FBI plays an integral role. Comey signed three of the four FISA warrant applications and renewals targeting former Trump campaign associate Carter Page, who was never charged with wrongdoing, during the Trump-Russia investigation. Comey defended the FBI’s use of the Steele dossier but admitted the dossier wasn't verified when he approved its use, and investigators were still working to verify its contents when he was fired in May 2017. The final FISA renewal happened a month later.

The FISA filings required approval from top members of the FBI and the Justice Department, so other possible targets of Horowitz's inquiry likely included the approvers of the four applications and renewals: Comey; Deputy Attorney General Sally Yates; Dana Boente, the only signatory in active government service and currently Trump’s top lawyer at the FBI; then-FBI Deputy Director Andrew McCabe; and then-Deputy Attorney General Rod Rosenstein, who appointed Mueller to be special counsel the month before.

Mueller told Congress he wasn't involved in approving that final FISA warrant.

The 412 pages of redacted FISA documents released in 2018 show the Justice Department and the FBI made extensive use of Steele’s dossier, which he put together in 2016 at the behest of the opposition research firm Fusion GPS. The Clinton campaign hired the firm through Marc Elias of the Perkins Coie law firm and was briefed about Steele's findings throughout the race.

Also last week, McCabe and former FBI special agent Peter Strzok, both fired during the Trump administration, separately sued the Justice Department alleging political bias played a role in their removals. Strzok demanded reinstatement and back pay, and McCabe demanded he receive his full retirement pension.

Horowitz has a history with both men. He previously concluded anti-Trump texts sent from Strzok to FBI lawyer Lisa Page, his mistress, not only showed Strzok was biased but also that he was willing to take official action to affect Trump's electoral prospects. Horowitz also concluded McCabe leaked information without authorization and "lacked candor" with investigators.

Horowitz's findings were cited in both their firings, and he's likely not done with them since both played prominent roles in the Trump-Russia investigation.

And last Thursday, FBI interview notes with top DOJ official Bruce Ohr shed light on his role as a conduit between Steele and the FBI in 2016 and 2017, after the bureau says it cut Steele off as a source for leaking to the media. Ohr told the FBI that Steele was desperate Trump not win and suggested some of Steele's information may have stemmed from "Russian conspiracy theories." Bruce Ohr's wife, Nellie, worked at Fusion GPS, and Bruce Ohr passed her research along to the FBI.

Horowitz is also likely looking into the FBI's use of an article by Michael Isikoff to bolster its FISA filings. The FBI told the court it didn't think the piece was connected to the dossier, even though Isikoff only wrote it after meeting with Steele.

The Justice Department inspector general said when he launched the investigation last year at the behest of then-Attorney General Jeff Sessions he would “examine the Justice Department’s and the FBI’s compliance with legal requirements" related to FISA filings against Page and review the DOJ’s and the FBI’s dealings with Steele. But while Horowitz said he would consider including other issues that arose during the investigation, Attorney General William Barr has since suggested the inquiry is narrow.

Former FBI General Counsel James Baker is also likely to be among those scrutinized, since he admitted he inserted himself into the FISA approval process and reviewed the first FISA application. Baker defended how the FBI handled the Steele dossier, claiming “we took it seriously” but “we didn’t necessarily take it literally.”

Republicans have argued the dossier's Democratic benefactors and its author's anti-Trump bias were left out of the FISA applications. Democrats countered that the FBI acted appropriately, saying the Justice Department and the FBI met the rigor, transparency, and evidentiary basis for probable cause.

Officials such as Deputy Assistant Secretary of State Kathleen Kavalec knew by October 2016 portions of Steele’s dossier were inaccurate. And State Department officials such as Jonathan Winer and Victoria Nuland seem to have been involved in spreading Steele's information within the U.S. government.

Many members of the media also received info from the Steele dossier, which may have contained Russian disinformation, at the behest of Fusion GPS founder Glenn Simpson.

The Steele dossier's central thesis was "a well-developed conspiracy of co-operation" between the Trump campaign and Russia, but Mueller didn't agree. Although Mueller concluded the Russians interfered in the 2016 presidential election, the investigation did not establish the Kremlin and Trump's campaign criminally conspired. Mueller’s report also shot down at least one of Steele’s biggest claims — that Trump lawyer Michael Cohen met with foreign hackers in Prague.

Horowitz reportedly “homed in” on Steele, and his team reportedly interviewed the ex-spy in June.

Meanwhile, Barr, along with U.S. Attorney John Durham, launched his own inquiry into the origins of the Trump-Russia investigation after Trump gave him broad declassification authority to look into it.