Mr. Durham has a track record with delicate cases where the investigative focus is on F.B.I. agents and C.I.A. officers. In 1999, Attorney General Janet Reno asked him to investigate the F.B.I.’s handling of James “Whitey” Bulger, a notorious mobster whom agents used as an informant. He secured convictions and unraveled a corrupt network of law enforcement officials working with Mr. Bulger.
Almost a decade later, Mr. Durham was directed to investigate the destruction of C.I.A. videotapes depicting the torture of detainees in secret prisons run by the agency. During that investigation, he interviewed Gina Haspel, now the director of the C.I.A., about her role in the destruction of the tapes. The investigation was expanded to include abuses of C.I.A. detainees. It ended with no criminal charges.
His most recent assignment involved investigating James A. Baker, the widely respected former top lawyer at the F.B.I., over a suspected leak of classified information. Mr. Durham quietly used agents with the United States Postal Service in that case because the Justice Department had decided that the F.B.I. could not investigate itself, people familiar with the investigation said. Mr. Baker has denied wrongdoing and was never charged with a crime.
For his review, Mr. Durham has enlisted Nora R. Dannehy, a veteran federal prosecutor who worked with him in Connecticut and led a two-year inquiry into whether department officials under President George W. Bush broke the law in firing several United States attorneys.
Many of the F.B.I. and C.I.A. officials that Mr. Durham is expected to attempt to interview have left government, including Bill Priestap, the bureau’s top counterintelligence agent during the Russia inquiry. Mr. Priestap privately told Congress last year that there was no F.B.I. conspiracy against Mr. Trump or his campaign.