A former FBI lawyer is being investigated for allegedly altering a document connected to the surveillance of Trump campaign adviser Carter Page, according to a report Thursday.

CNN reported the investigation, citing several sources briefed on the matter.

The FBI official was not identified nor is it clear exactly how the allegedly altered document influenced the investigation of Mr. Page. Sources told CNN the changes were significant enough to manipulate the document’s meaning.

The alleged changes were discovered as part of Justice Department Inspector General Michael Horowitz’s probe into allegations the FBI abused its power when applying for a surveillance warrant for Mr. Page.

Mr. Horowitz gave the document to John Durham, a U.S. Attorney who is leading the investigation into the origins of the FBI’s Russia probe. The document is at least one focus of Mr. Durham’s criminal probe, CNN reported.

A witness admitted to changing the document, CNN said.

The revelation is a shot in the arm for President Trump and his allies, who have argued that the FBI’s Russia probe was a politically-motivated effort to undercut his possible election.

Mr. Horowitz’s long-awaited report will be released on Dec. 9, Senate Judiciary Committee Chairman Lindsey Graham said Thursday.