Donald Trump has long attacked as a “hoax” and a “witch-hunt” the FBI’s investigation into ties between Russia and his 2016 presidential campaign.

On Friday night, he duly seized on news reports that an FBI lawyer is suspected of altering a document related to surveillance of former Trump campaign adviser Carter Page, alleging the FBI tried to “overthrow the presidency”.

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The allegation is part of a justice department inspector general review of the FBI’s Russia investigation, one of the most politically sensitive investigations in the history of the bureau which was ultimately taken over by special counsel Robert Mueller and resulted in charges against six Trump associates and more than two dozen Russians.

The justice department inspector general, Michael Horowitz, is expected to release his report on 9 December. Witnesses in the last two weeks have been invited in to see draft sections.

The report, centered in part on the use of a secret surveillance warrant to monitor the communications of a former Trump adviser, is likely to revive debate about an investigation that has shadowed Trump’s presidency since the beginning.

It will be released amid a House impeachment inquiry into Trump’s efforts to press Ukraine’s president to investigate Democratic rival Joe Biden at the same time military aid was being withheld.

Trump and his supporters are likely to seize on any findings of mistakes or bad judgment to support their claims of a biased investigation. Supporters of the FBI are likely to hold up as vindication any findings that the investigation was done by the book or free of political considerations.

The New York Times and the Washington Post have reported that the investigation is expected to find mistakes by lower-level officials within the FBI but will not accuse senior leaders of being motivated by political bias.

The allegation against the lawyer was first reported by CNN. The Post subsequently reported that the conduct of the FBI employee did not alter Horowitz’s finding that the surveillance application of Page had a proper legal and factual basis, though the lawyer was forced out.

“This was spying on my campaign – something that has never been done in the history of our country,” Trump told Fox & Friends on Friday. “They tried to overthrow the presidency.”

A person familiar with the case who was not authorized to discuss the matter by name and spoke to AP only on the condition of anonymity confirmed the allegation.

Spokespeople for the FBI and the inspector general declined to comment.

The FBI obtained a secret surveillance warrant in 2016 to monitor the communications of Page, who was never charged in the Russia investigation or accused of wrongdoing. The warrant, which was renewed several times and approved by different judges in 2016 and early 2017, has been one of the most contentious elements of the Russia investigation and was the subject of memos last year issued by Democrats and Republicans on the House intelligence committee.

Republicans have attacked the credibility of the warrant application since it cited information derived from a dossier of opposition research compiled by Christopher Steele, a former British spy whose work was financed by Democrats and the Hillary Clinton campaign.

“They got my warrant – a fraudulent warrant, I believe – to spy on myself as a way of getting into the Trump campaign,” Page said in an interview with Fox’s Mornings with Maria. “There has been a continued cover-up to this day. We still don’t have the truth, but hopefully, we’ll get that soon.“

The FBI director, Chris Wray, has told Congress he did not consider the FBI surveillance to be “spying” and that he has no evidence the FBI illegally monitored Trump’s campaign.

The attorney Ggeneral, William Barr, has said he believed “spying” did occur, but he also made clear at a Senate hearing earlier this year he had no specific evidence that any surveillance was illegal or improper. Barr has appointed the US attorney John Durham to investigate how intelligence was collected, and that investigation has since become criminal in nature, a person familiar with the matter has said.

Trump insists that members of the Obama administration “at the highest levels” were spying on his 2016 campaign.

“Personally, I think it goes all the way … I think this goes to the highest level,” he said in the Fox interview. “I hate to say it. I think it’s a disgrace. They thought I was going to win and they said, ‘How can we stop him?”