John H. Durham, the United States attorney in Connecticut, has a history of serving as a special prosecutor investigating potential wrongdoing among national security officials, including the F.B.I.’s ties to a crime boss in Boston and accusations of C.I.A. abuses of detainees.
His inquiry is the third known investigation focused on the opening of an F.B.I. counterintelligence investigation during the 2016 presidential campaign into possible ties between Russia’s election interference and Trump associates.