“I look forward to Bull Durham’s report — that’s the one I look forward to,” added Mr. Trump, who appointed Mr. Durham as the United States attorney for Connecticut in 2017.

The inspector general’s report makes no substantive reference to Mr. Durham’s investigation. But before the report’s release, Mr. Durham got into a sharp dispute with Mr. Horowitz’s team over a footnote in a draft of the report that seemed to imply that Mr. Durham agreed with all of Mr. Horowitz’s conclusions, which he did not, according to people familiar with the matter. The footnote did not appear in the final version of the report.

A former Justice Department investigator who knows both Mr. Barr and Mr. Durham, a Republican, said that while the men were aware of each other’s professional reputations, they are in no way close. Mr. Barr, who was unfamiliar with Mr. Durham’s recent work, made quiet inquiries before appointing him to lead the investigation, this person said.

The potential explosiveness of Mr. Durham’s mission was further underscored by the disclosure that he was examining the role of John O. Brennan, the former C.I.A. director, in how the intelligence community assessed Russia’s 2016 election interference.

Mr. Durham is known in New England’s close-knit law enforcement community for working long days on his own cases, and providing sought-after guidance on others’.

Wearing gunmetal-frame glasses and a drooping goatee, he rises early and dresses in the dark, often mismatching his suit jackets and pants. His reputation for discretion, on top of a long record of successful high-profile prosecutions, are among the reasons he has been a go-to person when Washington — under Republicans and Democrats alike — needs someone to handle sensitive tasks.

Mr. O’Connor, who was associate attorney general in 2008, was among those who recommended Mr. Durham lead an inquiry into the C.I.A.’s destruction in 2005 of videotapes depicting the torture of two operatives of Al Qaeda.