Mr. Barr and Mr. Durham decided that the tip was too serious and credible to ignore. But rather than assign it to another prosecutor, Mr. Barr had Mr. Durham investigate the matter himself — giving him criminal prosecution powers for the first time — even though the possible wrongdoing by Mr. Trump did not fall squarely within Mr. Durham’s assignment to scrutinize the origins of the Russia inquiry, the people said.

Mr. Durham never filed charges, and it remains unclear what level of an investigation it was, what steps he took, what he learned and whether anyone at the White House ever found out. The extraordinary fact that Mr. Durham opened a criminal investigation that included scrutinizing Mr. Trump has remained secret.


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But in October 2019, a garbled echo became public. The Times reported that Mr. Durham’s administrative review of the Russia inquiry had evolved to include a criminal investigation, while saying it was not clear what the suspected crime was. Citing their own sources, many other news outlets confirmed the development.

The news reports, however, were all framed around the erroneous assumption that the criminal investigation must mean Mr. Durham had found evidence of potential crimes by officials involved in the Russia inquiry. Mr. Barr, who weighed in publicly about the Durham inquiry at regular intervals in ways that advanced a pro-Trump narrative, chose in this instance not to clarify what was really happening.

By the spring and summer of 2020, with Mr. Trump’s re-election campaign in full swing, the Durham investigation’s “failure to deliver scalps in time for the election” began to erode Mr. Barr’s relationship with Mr. Trump, Mr. Barr wrote in his memoir.

Mr. Trump was stoking a belief among his supporters that Mr. Durham might charge former President Barack Obama and former Vice President Joseph R. Biden Jr. That proved too much for Mr. Barr, who in May 2020 clarified that “our concern of potential criminality is focused on others.”

Even so, in August, Mr. Trump lashed out in a Fox interview, asserting that Mr. Obama and Mr. Biden, along with top F.B.I. and intelligence officials, had been caught in “the single biggest political crime in the history of our country” and the only thing stopping charges would be if Mr. Barr and Mr. Durham wanted to be “politically correct.”