WASHINGTON — After a jet carrying Attorney General William P. Barr touched down in Rome last month, some diplomats and intelligence officials at the American Embassy were unsure why he had come. They were later surprised, two officials said, to discover that he had circumvented protocols in arranging the trip, where he met with Italian political and intelligence officials.

Everything about Mr. Barr’s visit was unusual — not least his companion and their mission: John H. Durham, a top federal prosecutor whom Mr. Barr has assigned to review the origins of the Russia investigation. They were seeking evidence that might bolster a conspiracy theory long nurtured by President Trump: that some of America’s closest allies plotted with his “deep state” enemies in 2016 to try to prevent him from winning the presidency.

After the special counsel, Robert S. Mueller III, testified to lawmakers in July that he found insufficient evidence to charge any Trump associates with conspiring with Russia, an emboldened Mr. Trump began pressing world leaders to cooperate with Mr. Barr.

Now, glimpses of the review are emerging. Mr. Barr is taking an active role, signaling that he has made it a priority and is personally overseeing it.