The FBI’s four wiretap affidavits to a federal judge in 2016 and 2017 deliberately hid a part of Trump campaign volunteer Carter Page’s professional life that would have revealed his patriotic role as a CIA informant instead of the bureau’s nefarious depiction, the Justice Department inspector general’s report shows.

The 476-page history of how the FBI targeted the Trump campaign contains over a dozen instances in which the FBI violated its own rules by including inaccuracies in the Foreign Intelligence Surveillance Act applications or omitting exculpatory evidence.

Perhaps none was more damaging than the actions of FBI attorney Kevin Clinesmith, whose social media posts revealed a deep dislike of President Trump.

During the drafting of FISA warrants in 2016 and 2017, the CIA confirmed to the FBI that Mr. Page, a Naval Academy graduate and energy investor who joined the Trump campaign as an adviser, served as an informant from 2008 to 2013.

That biographical fact never made it into any affidavit. In 2017, the CIA sent an email to the FBI restating that Mr. Page had been an asset. Mr. Clinesmith, who is not named in the inspector general’s report, altered the email to say Mr. Page was not an asset.

Sen. Lindsey Graham, South Carolina Republican, outed Mr. Clinesmith on Wednesday during the Senate Judiciary Committee hearing with Justice Department Inspector General Michael E. Horowitz.

Mr. Graham gave his theory about why Mr. Clinesmith “lied.”

“What does that matter?” Mr. Graham said. “Because if the court had known, then there is a lawful reason for Mr. Page to be talking to the Russian guy. He wasn’t working against his country. He was working with his country, which undercuts the idea he is a foreign agent. That’s why Clinesmith lied: because he didn’t want to stop this investigation.”

Rep. Mark Meadows, North Carolina Republican, first outed Mr. Clinesmith in June 2018 as one of five FBI officials — identified as FBI attorney 2 — called out in a previous inspector general’s report for bad-mouthing Mr. Trump.

To understand how Mr. Clinesmith’s deception harmed Mr. Page, the story starts in New York in 2013. The future Trump associate ran his own energy investment firm and was looking for opportunities in oil- and gas-rich Russia. He lived in Moscow in the mid-2000s while working as a Merrill Lynch banker and at that point apparently became a CIA source.

In New York, he met Victor Podobnyy at an energy symposium in January 2013. Mr. Page was working for U.S. intelligence but had been inactive for a year. Moscow posted Mr. Podobnyy as a United Nations diplomat, but he actually was a spy for the SVR, the Russian CIA. Mr. Podobnyy and two countrymen ran an espionage ring and tried to recruit Mr. Page as he sought help in making money via Russian energy.

Mr. Podobnyy, whose calls were intercepted by the FBI, according to a Justice Department filing in 2015, played along with Mr. Page’s desire to find a business deal in Russia. The recruitment ended in June 2013 when FBI agents interviewed Mr. Page, who is identified as “Male 1” in court documents.

One of the three Russian spies was convicted of espionage. Mr. Podobnyy and a comrade had left the U.S. by the time the government filed charges.

Mr. Page cooperated. He was never charged.

Fast-forward to the fall of 2016, when the FBI and its Crossfire Hurricane team were drafting what would be the first of four FISA warrants that would allow the government to electronically spy on Mr. Page for a year. Agents also could monitor his activities for the campaign. Emails, texts and documents would all be in play.

Included in all four warrant applications was Mr. Page’s contact with Mr. Podobnyy. The Justice Department released the heavily redacted application in 2018 under pressure from Rep. Devin Nunes, a California Republican who then led the House Permanent Select Committee on Intelligence. Mr. Nunes accused the FBI of violating its own FISA rules. The inspector general’s report released Monday supported his allegations.

Altered document

By September 2016, the FBI was finishing the first warrant on Mr. Page. Its prime piece of evidence was the now-discredited Christopher Steele dossier, sourced to the Kremlin and financed by the Democratic National Committee and the Hillary Clinton presidential campaign.

An FBI attorney for the office of intelligence asked whether Mr. Page, as he claimed, had worked for other intelligence agencies. A Crossfire Hurricane agent said that perhaps, briefly, when he lived in Moscow. Based on that response, the attorney did not include that part of Mr. Page’s biography.

The agent’s information was inaccurate.

An Aug. 17, 2016, memo from the CIA (the inspector general’s report describes it simply as another agency) said Mr. Page had been approved as an operational contact in 2008 and reported his contacts with Russian intelligence agents in 2009.

When agents interviewed Mr. Page in 2013, he told them he had been a CIA source for years.

The FBI included none of this in the FISA applications, as it should have been, the inspector general said.

When news leaked in June 2017 that the FBI had bugged Mr. Page, he went public to deny the Steele dossier’s allegations against him. He also said he had been a U.S. intelligence source.

At that point, Mr. Clinesmith (identified in the inspector general’s report as the FBI office of general counsel, or OGC, attorney) sent an email to the CIA asking for confirmation. The CIA answered that the FBI should review its August 2016 memorandum and that Mr. Page did indeed have a working relationship.

Here is where Mr. Clinesmith’s skullduggery occurred, according to the inspector general: “The OGC attorney told [the FISA agent] that Page had never had a relationship with the other U.S. government agency. In addition, the OGC attorney altered the email that the other U.S. government agency had sent to the OGC attorney so that the email stated that Page had not been a source for the other agency. The OGC attorney then forwarded the altered email to [the FISA agent], who told us he relied on the email.”

Afterward, the agent completed the last of four, three-month FISA warrants without including Mr. Page’s CIA work.

The office of intelligence attorney told the inspector general, “It would have been a significant fact if Page had a relationship with the other U.S. government agency that overlapped in time with his interactions with known Russian intelligence officers described in the FISA applications because it would raise the issue of whether Page interacted with the Russian intelligence officers at the behest of the other agency or with the intent to assist the U.S. government.”

The New York Times reported that Mr. Clinesmith is under criminal investigation by John Durham, the U.S. attorney for Connecticut. Attorney General William P. Barr named Mr. Durham to investigate the origins of the FBI Russia probe. Mr. Barr told NBC News on Tuesday that he disagrees with Mr. Horowitz’s conclusion that the investigation into the Trump campaign was justified.

Mr. Clinesmith moved from the FBI’s office of general counsel to special counsel Robert Mueller’s probe. He resigned in 2018 after Mr. Horowitz discovered the document doctoring.

Mr. Meadows identified Mr. Clinesmith as one of the FBI employees who bashed Mr. Trump in instant messages captured in the Horowitz report on the Clinton email investigation.

Mr. Clinesmith messaged after Mr. Trump won the election: “I just can’t imagine the systematic disassembly of the progress we made over the last 8 years. ACA is gone. Who knows if the rhetoric about deporting people, walls, and crap is true. I honestly feel like there is going to be a lot more gun issues, too, the crazies won finally. This is the tea party on steroids. And the GOP is going to be lost, they have to deal with an incumbent in 4 years. We have to fight this again. Also Pence is stupid.”

Making a reference to his role in Crossfire Hurricane, Mr. Clinesmith said, “Plus, my god damned name is all over the legal documents investigating his staff.”

Mr. Clinesmith said his personal political beliefs didn’t affect the way he investigated the Trump campaign.