Former U.S. Attorney Guy Lewis predicted the Justice Department inspector general's report on alleged Foreign Intelligence Surveillance Act abuses will be "hard-hitting," with multiple recommendations for prosecution.

During an interview Monday on Fox News, Lewis said he used to work with Inspector General Michael Horowitz at the Justice Department and called him as "tough as a nail." Lewis added, "He’s tough, he is smart, and I think the report will be a blockbuster." He expects Horowitz's findings to make special counsel Robert Mueller's report on Russian interference in the 2016 election "look like a sixth grade book report."

Asked to explain why, Lewis said, "It’s going to come out. It’s going to be hard-hitting. [Horowitz] spent almost two years interviewing various people — witnesses. Looking at what he describes [as] almost a million documents. And I think he’s going to make recommendations regarding prosecution and turn this whole FISA — this Foreign Intelligence Surveillance Act — the submissions, I think he will turn them on their head."

British ex-spy Christopher Steele's anti-Trump dossier, which was funded by the Clinton campaign and the Democratic National Committee, was used by the FBI to obtain FISA warrants to surveil one-time Trump campaign adviser Carter Page, an American citizen who was never charged with any wrongdoing.

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

[Analysis: Who and what the Horowitz report is likely to cover]

Although some Trump allies have expressed high expectations for Horowitz's report and Attorney General William Barr's review of the origins of the Russia investigation, others are preaching caution. "Republicans usually set the bar so high that when we don't meet it, the media narrative then becomes Republicans don't even own their promises. Just promise a fair investigation that goes wherever the facts take us," former South Carolina Rep. Trey Gowdy said on Fox News last month.

Lewis, who served with Barr at the Justice Department under President George H.W. Bush, was reacting to an interview Senate Judiciary Committee Chairman Lindsey Graham did over the weekend in which he said he believes the IG report would be “ugly and damning” for the DOJ and its handling of the Russia investigation.

Graham said he wants "as much as possible" declassified from Horowitz's report as possible, but former New Hampshire Gov. John Sununu warned former FBI Director Christopher Wray might fight to keep its contents classified because he has been "co-opted by the FBI bureaucracy."

Lewis said he does not agree with Sununu's assessment, calling Wray a "good, honest broker," but advised the FBI chief to "err on the side of transparency" because he suspects the FISA Court may have been used by the FBI to target President Trump's 2016 campaign.

Asked where the investigation is headed, Lewis said former FBI officials Peter Strzok and Andrew McCabe ought to be "worried" about submissions that were made to the FISA Court. Both Strzok and McCabe filed separate lawsuits against the DOJ and FBI claiming they were subject to politically motivated firings last year.

He also said former Trump campaign adviser George Papadopoulos and former national security adviser Michael Flynn should be be "sort of happy" because Horowitz's report might be used to help them overturn their convictions, stemming from Mueller's investigation, for lying to FBI investigators.