The trip to Italy generated criticism that Mr. Barr was doing the president’s bidding and micromanaging a supposedly independent investigation. But Mr. Barr seems to have embraced his role, signaling that he has made the investigation a priority and is personally overseeing it.

Now, glimpses of the Durham review are emerging. Investigators have interviewed F.B.I. officials about their work in 2016, examined intelligence files from around that time and cast a wide net in setting up interviews with a foreign cast of characters who played disparate roles in the pre-election drama…

Mr. Barr has portrayed the review as an attempt to ferret out any abuse of power by law enforcement or intelligence officials. But it is also a politically charged effort that takes aim at the conclusions of the American law enforcement and intelligence communities about Russia’s election interference based on years of work by multiple agencies.

The review could fray diplomatic relations with overseas partners and affect Mr. Trump’s political fortunes. And it is testing traditional boundaries drawn to keep the powers of American law enforcement out of electoral politics.