An FBI lawyer is under criminal investigation for allegedly altering a document related to the surveillance of onetime Trump campaign adviser Carter Page.

The lower-level line attorney, who is no longer with the bureau, is being scrutinized by U.S. Attorney John Durham as part of his expansive criminal investigation into the origins of the Russia investigation, according to CNN.

Justice Department Inspector General Michael Horowitz uncovered the document as part of his investigation into alleged Foreign Intelligence Surveillance Act abuses, which concluded in September, and has been in contact with Durham. The Connecticut-based prosecutor's administrative review, overseen by Attorney General William Barr, turned into a criminal investigation this fall, giving Durham the power to impanel a grand jury and hand down indictments.

No charges relevant to the situation appear to have been filed in court. A Justice Department representative did not immediately return a request for comment.

Horowitz's team interviewed the FBI lawyer, who admitted to making a change to the document that was substantive enough to twist its meaning.

The attorney was found to have wrongly asserted having documentation to support a claim in talks with the Justice Department about the factual basis for the application and then provided a falsified email to back it up, according to the Washington Post. A draft of Horowitz's report said this employee's conduct did not affect the overarching validity of the application to electronically surveil Page, but the attorney was forced out of the FBI after the episode came to light.

"If there was an FBI agent sworn to uphold the Constitution who can be proven to have altered the document in connection with a legal proceeding, including the obtaining of a FISA warrant, that's really serious. It doesn't get a lot more serious than that," former U.S. Attorney Preet Bharara said Thursday evening on CNN.

In his letter to Congress announcing the completion of his investigation, Horowitz said his "team has reviewed over one million records and conducted over 100 interviews."

Horowitz confirmed Thursday that he expects his FISA report to be released to the public on Dec 9 and is scheduled to testify before the Senate Judiciary Committee two days later. The inspector general has predicted minimal redactions after a classification review and final session for witnesses to provide feedback.

Republicans believe Horowitz's report will reveal an effort to undermine President Trump's 2016 campaign in which the FBI misled the FISA Court in its reliance on an unverified dossier compiled by British ex-spy Christopher Steele, whose research about Trump and his associates was partially funded by Hillary Clinton's 2016 campaign and the Democratic National Committee through the Perkins Coie law firm. Some have even predicted indictments and prison time for officials who signed off on the warrants.

Democrats, as well as current and former FBI officials, have dismissed allegations of wrongdoing and have raised concerns that information about U.S. intelligence-gathering could be leveraged to discredit former special counsel Robert Mueller.

The FISA application and three renewals targeting Page required the approval of top members of the FBI, the DOJ, and the Foreign Intelligence Surveillance Court, but was also handled by lower-level officials. The initial warrant application was approved in October 2016, and the final renewal came in June 2017.

A line attorney involved with the Page FISA process was discussed by Sally Moyer, a unit chief in the FBI Office of the General Counsel, during her 2018 testimony for a House-led investigation into the apparent wrongdoing at the FBI and Justice Department. She described how "the line attorney approved it to go to [the DOJ Office of Intelligence], but I agreed with that decision." Moyer said "that is accurate" when asked if the line attorney did not believe approving the Page FISA application was a close call and stated "that's correct" to the question of whether the line attorney believed there was probable cause to support it.

Moyer also said she believed there was a "50/50" chance the FISA application would be approved without Steele's dossier of allegations on Trump's ties to Russia.

[READ MORE: The Carter Page FISA: A timeline]

The application process involved officials as high up as former FBI Director James Comey, who was fired by Trump in May 2017. Comey said in September he is "highly confident" that he will not be indicted.

Former FBI Deputy General Counsel Trisha Anderson described to lawmakers in 2018 the “unusual” way the surveillance request targeting Page was handled by top leadership at the Justice Department and the bureau. She specifically named Deputy Attorney General Sally Yates, former FBI Deputy Director Andrew McCabe, and former FBI General Counsel James Baker.

Baker, who took a lead role in reviewing the Page FISA applications before submitting them to the FISA Court, admitted in June that he was cooperating with Horowitz's investigation. A month prior, Baker, who is now a CNN legal analyst, said he took the salacious allegations in Steele's dossier "seriously" but "not necessarily literally," and also he was "nervous" about the inspector general.

Baker and McCabe, who is a CNN contributor, appeared together on the cable news network Thursday evening to discuss impeachment, but were first asked to address the FISA investigation report. Baker said he has not reviewed Horowitz's report. McCabe said he is in the process of reviewing it. "I’ve agreed not to speak about it in any way until that process is over," he said.

Under suspicion of being a Russian agent, Page became a subject of interest in the FBI's counterintelligence investigation, called Crossfire Hurricane, which began in July 2016. and was later wrapped into Mueller's investigation. Page was never charged with a crime as part of Mueller's investigation, which failed to establish criminal conspiracy between the Trump campaign and the Kremlin, and denied being an agent for Russia.

Last summer, the Justice Department took the unprecedented step of releasing more than 400 redacted pages of top-secret documents on the FISA warrant obtained to wiretap Page after Trump declassified their existence.

Page has been heavily critical of the Russia investigation and the media coverage of it, which he claimed was an effort to advance a narrative for impeachment. "Unfortunately, it was completely spun that as you're correctly alluding to in this fake Mueller report, they just refer to this as oh, he's, you know, colluding, if you will, with these Russian intelligence officers," he said in June.

The U.S. intelligence community concluded in 2017 that Russia was responsible for hacking thousands of Democratic emails and providing those stolen records to WikiLeaks for dissemination, a claim bolstered through Mueller’s investigation. Independent investigations by the House and Senate also concluded the Kremlin interfered in the 2016 presidential election.