A former FBI lawyer charged with altering a key document used in connection with the FBI's surveillance of former Trump campaign aide Carter Page has agreed to plead guilty on Friday to making a false statement.

The charge against Kevin Clinesmith marks the first criminal case brought by U.S. Attorney John Durham's yearlong probe into the origins of the 2016 Russian election meddling investigation, an inquiry President Donald Trump and his allies derided as a "hoax."

Durham was picked by U.S. Attorney General William P. Barr to probe possible misconduct by federal authorities involved in the FBI's efforts to examine links between Trump's campaign and Russia during Trump's campaign against Democrat Hillary Clinton.

Clinesmith, who worked for the FBI from 2015 to 2019, is expected to admit in federal court in Washington, D.C., that he altered a June 2017 email from the CIA in a way that suggested Page, an admitted CIA asset, was not a "source" for the agency. Relying on the falsified document, the FBI then applied for its fourth surveillance warrant on Page, according to court documents. The alteration was brought to light last year by the Justice Department's inspector general, Michael Horowitz, who referred Clinesmith for criminal prosecution.

Although the Page surveillance took place after he had left the Trump campaign and was part of the broader FBI examination of Russian election meddling, Trump and his allies have seized on it as evidence that the administration of former President Barack Obama "spied" on his election team.

Calling Clinesmith a "corrupt FBI attorney," Trump told White House reporters on Friday that it was just the beginning of more to come from Durham's investigation.

"The fact is they spied on my campaign and they got caught, and you'll be hearing more," Trump said.

In an interview with Fox News' Sean Hannity on Thursday, Barr described the Clinesmith plea as "an indication that things are moving along at the proper pace as dictated by the facts in this investigation."

While Barr said he is "not going to do anything inappropriate before the election," he suggested that he would likely release Durham's findings ahead of the November vote.

The Russia probe was launched in July 2016 shortly after the FBI received a tip from Australian intelligence that George Papadopoulos, then a foreign policy adviser for the Trump campaign, had suggested that Russia had approached the Trump people with offers of help in the form of releasing thousands of embarrassing emails hacked from Democrats.

In addition to Page and Padopoulos, the FBI investigated two other Trump campaign operatives. In the end, while special counsel Robert Mueller uncovered numerous contacts between the Trump campaign and Russia, he concluded that there wasn't enough evidence to prove a conspiracy.

Little is known about what Durham has unearthed or whether anyone else will be charged. While Barr has made clear neither Obama nor former vice president and current Democratic presidential candidate Joe Biden is under investigation, he has said Durham has uncovered "troubling" evidence of misconduct.

Barr, who became Trump's second attorney general last year, has faced criticism over publicly discussing the Durham investigation and refusing to commit to not releasing his findings before the Nov. 3 presidential election pitting Trump against Biden.

Justice department policies prohibit officials from commenting publicly making on ongoing investigations and taking steps "for the purpose of affecting any election, or for the purpose of giving an advantage or disadvantage to any candidate or political party."

In recent months, the attorney general has stepped up his attacks on the Russia investigation, saying in an April interview with Fox that it was carried out "without any basis" and calling it "one of the greatest travesties in American history." During a July 29 congressional appearance, Barr referred to the Russia investigation as "the bogus Russiagate."

Mueller has pushed back against claims that the investigation was "illegitimate," writing in the Washington Post in July that the probe was "of paramount importance."

"Russian efforts to interfere in our political system, and the essential question of whether those efforts involved the Trump campaign, required investigation," Mueller wrote.

In a report released last year, the Justice Department's inspector general concluded the FBI investigation had been "predicated" on intelligence about Russian interference.

Barr said he disagreed with some of the report. And in an extraordinary move for a prosecutor, Durham released a statement in response to the IG report saying "we do not agree with some of the report's conclusions as to predication and how the FBI case was opened."