Federal prosecutors reviewing the origins of the Russia investigation have asked witnesses pointed questions about any anti-Trump bias among former F.B.I. officials who are frequent targets of President Trump and about the earliest steps they took in the Russia inquiry, according to former officials and other people familiar with the review.

The prosecutors, led by John H. Durham, the United States attorney in Connecticut, have interviewed about two dozen former and current F.B.I. officials, the people said. Two former senior F.B.I. agents are assisting with the review, the people said.

The number of interviews shows that Mr. Durham’s review is further along than previously known. It has served as a political flash point since Attorney General William P. Barr revealed in the spring that he planned to scrutinize the beginnings of the Russia investigation, which Mr. Trump and his allies have attacked without evidence as a plot by law enforcement and intelligence officials to prevent him from winning the 2016 election.