The president granted Mr. Barr sweeping powers for the review, though he did not open it as a criminal investigation. That means he gave Mr. Durham the power only to read materials the government had already gathered and to request voluntary interviews from witnesses, not to subpoena witnesses or documents. It is not clear whether the status of the review has changed.

Mr. Durham’s investigators appeared focused at one point on Mr. Strzok, said one former official who was interviewed. Mr. Strzok opened the Russia inquiry in late July 2016 after receiving information from the Australian government that the Russians had offered damaging information on Hillary Clinton to a Trump campaign adviser. Mr. Durham’s team has asked about the events surrounding the Australian tip, some of the people familiar with the review said.

Mr. Durham’s team, including Nora R. Dannehy, a veteran prosecutor, has questioned witnesses about why Mr. Strzok both drafted and signed the paperwork opening the investigation, suggesting that was unusual for one person to take both steps. Mr. Strzok began the inquiry after consulting with F.B.I. leadership, former officials familiar with the episode said.

Mr. Durham has also questioned why Mr. Strzok opened the case on a weekend, again suggesting that the step might have been out of the ordinary. Former officials said that Mr. McCabe had directed Mr. Strzok to travel immediately to London to interview the two Australian diplomats who had learned about the Russians’ offer to help the Trump campaign and that he was trying to ensure he took the necessary administrative steps first.

It is not clear how many people Mr. Durham’s team has interviewed outside of the F.B.I. His investigators have questioned officials in the Office of the Director of National Intelligence but apparently have yet to interview C.I.A. personnel, people familiar with the review said. Mr. Durham would probably want to speak with Gina Haspel, the agency’s director, who ran its London station when the Australians passed along the explosive information about Russia’s offer of political dirt.

Many of the questions from Mr. Durham’s team overlapped with ones that the Justice Department inspector general, Michael E. Horowitz, has posed in his own look into aspects of the Russia inquiry, according to the people.