In a highly anticipated report released Monday, Justice Department Inspector General Michael Horowitz determined that the FBI had adequate cause in July 2016 to justify opening an investigation into links between the Trump campaign and Russia and did not find hard evidence of political bias in that decision, undermining allegations by President Trump and his allies that the bureau acted improperly in launching the probe.

But Horowitz did find serious flaws in the application process for Foreign Intelligence Surveillance Act (FISA) warrants to surveil Donald Trump campaign aide Carter Page, which will provide new grist for the president in his complaints about the investigation into his campaign and whether members were coordinating, wittingly or unwittingly, with Russia to interfere in the 2016 election.

“We did not find documentary or testimonial evidence that political bias or improper motivation influenced” the decision to open the FBI investigation, called ‘Crossfire Hurricane,’ the report says. “We found that Crossfire Hurricane was opened for an authorized investigative purpose and with sufficient factual predication.”

While Attorney General William Barr praised the Inspector General’s work, he offered a contradictory view on that key part of Horowitz’s findings, saying in a statement, “The Inspector General’s report now makes clear that the FBI launched an intrusive investigation of a U.S. presidential campaign on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken.”

Connecticut U.S. Attorney John Durham, who is undertaking a separate investigation into some intelligence collection activities on the Trump campaign, also released a statement disputing that finding. “Based on the evidence collected to date, and while our investigation is ongoing, last month we advised the Inspector General that we do not agree with some of the report’s conclusions as to predication and how the FBI case was opened,” he said.

Trump has long claimed that top FBI and Justice Department officials acted out of bias against him when they investigated his campaign’s possible links to Russia. “I predict you will see things that you don’t even believe, the level of corruption—whether it’s Comey; whether it’s Strzok and his lover, Page; whether it’s so many other people—McCabe; whether it’s President Obama himself,” Trump said in October of Horowitz’s findings.

Speaking Monday afternoon about an hour after the report was publicly released, Trump said he had been “briefed” on it and said he viewed the conduct outlined within it as “an attempted overthrow.” “They fabricated evidence and they lied to the courts,” he said.

While the top-line conclusion of Horowitz’s report is a stunning rebuke of the claims of corruption and bias, Horowitz did unearth major failures in the FISA applications— to the point that he is recommending a review of how the FBI conducts FISA surveillance on American citizens. Overall, Horowitz found 17 “significant inaccuracies and omissions” over the course of four FISA applications for Page. “Members of the Crossfire Hurricane team failed to meet the basic obligation to ensure that the Carter Page FISA applications were ‘scrupulously accurate,’” the report says. In the preparation of these applications, “the Crossfire Hurricane team failed to comply with FBI policies, and in so doing fell short of what is rightfully expected from a premier law enforcement agency entrusted with such an intrusive surveillance tool,” the report says.

Horowitz found that the original FISA application relied in large part on information contained in the dossier compiled by former British intelligence officer Christopher Steele, and that some of that key information, despite being either uncorroborated or inaccurate, continued to show up in the applications for renewals. “As the FBI obtained additional information raising significant questions about the reliability of the Steele election reporting, the FBI failed to reassess the Steele reporting relied upon in the FISA applications, and did not fully advise NSD or OI officials,” the report says. All of the factual inaccuracies taken together “resulted in FISA applications that made it appear that the information supporting probable cause was stronger than was actually the case,” the report says.

Horowitz also reviewed the use of confidential sources and undercover employees in the Crossfire Hurricane operation, noting that there were multiple recorded interactions between these contacts, Carter Page, Trump campaign adviser George Papadopoulous and another “high-level Trump campaign official.” Barr had said previously that he believed “spying” occurred on the Trump campaign, and Horowitz determined that there was no improper use of these types of informants during the investigation.

After investigating for more than a year, Horowitz will appear before the Senate Judiciary Committee to testify on Wednesday about his findings, prompting concerns that his testimony will be co-opted in the bitterly partisan fight over the ongoing impeachment inquiry into the President. With a mixed-bag of findings that both show there was adequate justification to launch the investigation in the first place, while also sharply criticizing the handling of key surveillance processes, both Republicans and Democrats will find aspects of the report to support their messaging.

“Whatever he finds and however he describes it, I worry that the rhetoric will be spun and amplified inaccurately,” says David Kris, founder of Culper Partners consulting firm who served as the assistant attorney general for DOJ’s National Security Division from 2009 to 2011. “The Trump Administration has been extreme in trying to influence investigations and in selectively representing their outcomes.”

In a career as Inspector General full of tough investigations, this could be Horowitz’s trickiest because of the political significance of probing government surveillance on a presidential campaign less than a year before national elections. But this is not his first high stakes political investigation, and he has maintained a tough, even-handed reputation over his seven years on the job, colleagues say. “He has a reputation as a very passionate IG,” says Shanlon Wu, who worked with Horowitz at the Justice Department under Attorney General Janet Reno. “He’s taken on a lot of difficult investigations, he’s butted heads some with the FBI at times… [He] really, really pushed things and was not afraid to venture where other people might be loath to venture.”

Horowitz has been the Justice Department’s inspector general since 2012. Before that, was a partner at the law firm Cadwalader, Wickersham & Taft, after working at the Justice Department and as an Assistant U.S. Attorney for the Southern District of New York earlier in his career. During his time as inspector general, he’s earned the respect of his peers: the government’s 73 other inspectors general have elected him to three terms as chairman of the Council of the Inspectors General on Integrity and Efficiency. And those who have seen him work say he approaches sensitive investigations carefully, meticulously and thoroughly.

Wu remembers when he used to attend meetings with Horowitz when Horowitz was the chief of staff for the assistant attorney general in charge of the Criminal Division, other attendees would be taking “reams and reams” of notes, while Horowitz would bring only one single note card with him. “It was very methodical,” Wu recalls. “Kind of minimalist.” Robert Storch, the current Inspector General for the National Security Agency who was formerly Horowitz’s deputy in the IG’s office at the Justice Department, says that Horowitz is hands-on with the details of his investigations and reviews. “He’d always want to see the underlying documentation and satisfy himself regarding what it showed,” Storch says.

In his first year as inspector general, Horowitz waded into the politically charged gun-trafficking case called Operation Fast and Furious. During the operation, which ran from 2009 to 2011, agents in the Bureau of Alcohol, Tobacco, Firearms and Explosives lost track of hundreds of illegal weapons they had let go in hopes of tracking them and bringing a bigger case. Many Republicans blamed Barack Obama’s Attorney General Eric Holder for the scandal, but Horowitz largely cleared him of wrongdoing, and found no evidence to support the theory that senior Obama officials had approved the operation. Still, Horowitz referred more than a dozen other people at the Justice Department and ATF for possible punishment in the operation that Horowitz wrote was rife with “a series of misguided strategies, tactics, errors in judgment and management failures.” He has also issued reports finding persistent issues with FBI retaliation against internal whistleblowers and FBI failure to investigate employees who failed polygraph tests.

Read more: The FBI Is in Crisis. It’s Worse Than You Think

He has a fine-tuned sense of the political pressures at play, colleagues say. Over his years working in Washington, one former colleague says Horowitz, a former prosecutor, has “internalized the lessons of D.C.” in a way that serves him well. “You can only be effective as a down-the-middle prosecutor if you understand the various political influences that might knock you off that middle course,” the former colleague says. “He understands how he’s got to be careful.”

In April 2018, Horowitz wrote a scathing report about ousted former FBI Deputy Director Andrew McCabe, finding McCabe had improperly orchestrated a leak to the media about Hillary Clinton’s family foundation less than two weeks before the election, and then lied to internal investigators about it. The Department is still weighing whether to criminally charge McCabe, while Trump often invokes McCabe as a lightning rod in his complaints about a perceived conspiracy against him in the Justice Department and intelligence community.

Months later, Horowitz discovered anti-Trump text messages that had been sent between an FBI agent overseeing the investigation into links between Trump’s campaign and Russia and an FBI lawyer with whom he was having an affair. Horowitz determined there wasn’t evidence that this anti-Trump sentiment had affected the investigation, but the texts were explosive, and Peter Strzok, the agent, was fired. The discovery of the Strzok and Lisa Page texts spurred Horowitz to expand his investigation to include looking into the origins of the investigation, but he describes in the latest report that Strzok and Page did not play pivotal roles in the decision to open the investigation. Trump often still invokes Strzok and Page at political rallies and in comments to reporters, mocking them and holding their text messages as more evidence of bias against him in the Justice Department.

Also in the summer of 2018, Horowitz completed an 18-month investigation that found former FBI Director James Comey and other top officials hadn’t followed standard procedures in their handling of the investigation into Hillary Clinton’s email server. In a similar pattern to what he determined in this latest report, Horowitz said he did not find evidence of political bias in Comey’s motives, but he didn’t pull punches on what he found about Comey’s troubling behavior. He said Comey’s decision-making “violated long-standing [Justice] Department practice.” “The decisions negatively impacted the perception of the FBI and the Department as fair administrators of justice,” the report concluded.

Throughout the past seven years, and perhaps increasingly so under Trump, Horowitz has walked a delicate tightrope, and this latest report may test his power like never before. “One of the things about IGs you really have to be careful about, is obviously you can’t be swayed by partisan influences,” says Storch. “But having said that, you can’t even give the appearance of that. Because if there was even a suggestion that your conclusions were shaded one way or another, you might as well close the shop.”

Get our Politics Newsletter. The headlines out of Washington never seem to slow. Subscribe to The D.C. Brief to make sense of what matters most. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Write to Tessa Berenson at tessa.berenson@time.com.