Legal experts were divided on whether Mr. Mueller’s team should have sought to question Mr. Dowd. The investigators compiled substantial evidence that Mr. Trump tried to obstruct justice even without Mr. Dowd’s testimony, and an attempt to interview him could have set off a lengthy court battle with an uncertain outcome.

“Given all that Mueller’s team had on their plate, it doesn’t strike me as unreasonable for them to have said, ‘This is not what we want to spend our time on,’” said David A. Sklansky, a Stanford law professor and a former criminal prosecutor.

But questioning Mr. Dowd about whether Mr. Trump wanted him to dangle pardons or other favorable treatment to witnesses might have been a worthy investigative pursuit because it would have cut to the heart of whether the president abused his power.

Joyce Vance, a professor at the University of Alabama School of Law and a former United States attorney, said she would have wanted to use all tools available to learn whether Mr. Trump told Mr. Dowd to make the call.