Washington (CNN) A former FBI lawyer is under criminal investigation after allegedly altering a document related to 2016 surveillance of a Trump campaign adviser, several people briefed on the matter told CNN.

Yet the situation did not sway an independent Justice Department watchdog from finding the surveillance was valid, sources said.

Still, the possibility of a substantive change to an investigative document is likely to fuel accusations from President Donald Trump and his allies that the FBI committed wrongdoing in its investigation of connections between Russian election meddling and the Trump campaign.

The findings are expected to be part of Justice Department Inspector General Michael Horowitz's review of the FBI's effort to obtain warrants under the Foreign Intelligence Surveillance Act on Carter Page, a former Trump campaign aide. Horowitz will release the report next month.

Horowitz turned over evidence on the allegedly altered document to John Durham, the federal prosecutor appointed early this year by Attorney General William Barr to conduct a broad investigation of intelligence gathered for the Russia probe by the CIA and other agencies, including the FBI. The altered document is also at least one focus of Durham's criminal probe.