A former FBI lawyer plans to plead guilty to falsifying a claim made to sustain government surveillance on a key figure in former special counsel Robert Mueller’s investigation of Russian meddling in the 2016 election, according to court documents.

It is the first legal development to come from a review of the Mueller team’s work, an effort led by John Durham, the U.S. attorney in Connecticut. Attorney General William Barr assigned him more than a year ago to examine the origins of the Russia investigation.

A legal filing submitted Friday in federal court in Washington, D.C., said Kevin Clinesmith will plead guilty to a single charge of making a false statement by altering an e-mail in the course of seeking a renewal of government surveillance of Carter Page, a former Trump campaign adviser. The warrant for approval from the Foreign Intelligence Surveillance Court has been a flashpoint for conservative critics of the FBI and the Mueller investigation.

The Justice Department’s inspector general reported last December than when Clinesmith was working in the FBI’s Office of General Counsel, he altered an e-mail about Page so that it said he was “not a source” for another U.S. intelligence agency. Page has publicly said he briefly was a source for the CIA.

Clinesmith told the inspector general he did not consider Page to be a “recruited asset.”

“Kevin deeply regrets having altered the e-mail,” said his lawyer, Jonathan Shur. “It was never his intent to mislead the court or his colleagues, as he believed the information he relayed was accurate, but Kevin understands what he did was wrong and accepts responsibility.”

Page said in a statement Friday that Clinesmith was “finally being held accountable and pleading guilty to committing a felony for his involvement in the plot to falsely portray me and by implication the Trump administration as traitors. The actions by the full band of government officials and Democrat operatives involved in the creation of the false applications for my FISA surveillance warrants were entirely unconscionable.”

“Clinesmith, his organization and their associates put my very life at risk, leading to abusive calls and death threats because of my personal opinions and support for President Trump,” Page continued. “There is a long way to go on the road to restoring justice in America, but certainly a good first step has now been taken.”

President Donald Trump remarked on the development at a White House news conference Friday, calling Clinesmith “a corrupt FBI attorney who falsified FISA warrants in James Comey’s corrupt FBI.”

“That is just the beginning, I would imagine, because what happened should never happen again,” Trump said. “He is pleading guilty. It’s a terrible thing. The fact is, they spied on my campaign and they got caught. You will be hearing more.”

Barr foreshadowed the expected plea agreement in an interview with Fox News on Thursday evening. He said it not be an “earth-shattering development,” but would be “indication that things are moving along at the proper pace, as dictated by the facts in this investigation.”

The inspector general’s report concluded that the FBI had a legitimate reason for opening an investigation in to Russian election meddling and whether anyone connected with the Trump campaign was involved. The report concluded that there was no proof of political bias in the decision.

But Inspector General Michael Horowitz said the FBI made serious and repeated mistakes in seeking an order under the Foreign Intelligence Surveillance Act to conduct surveillance of Page. The FBI’s submissions to the court made assertions that were “inaccurate, incomplete, or unsupported by appropriate documentation,” he said.

Barr said he asked Durham to look into the origins of the investigation because he did not believe the FBI had a proper reason for launching it. He told NBC News last December that the FBI started looking at the campaign on the thinnest of suspicions and kept pushing even after it went nowhere.

“There has to be some basis before we use these very potent powers in our core first amendment activity. And here, I felt this was very flimsy,” he said.