WASHINGTON — It became a regular litany of grievances from President Donald J. Trump and his supporters: The investigation into his 2016 campaign’s ties to Russia was a witch hunt, they maintained, that had been opened without any solid basis, went on too long and found no proof of collusion. Egged on by Mr. Trump, Attorney General William P. Barr set out in 2019 to dig into their shared theory that the Russia investigation likely stemmed from a conspiracy by intelligence or law enforcement agencies. To lead the inquiry, Mr. Barr turned to a hard-nosed prosecutor named John H. Durham, and later granted him special counsel status to carry on after Mr. Trump left office. But after almost four years — far longer than the Russia investigation itself — Mr. Durham’s work is coming to an end without uncovering anything like the deep state plot alleged by Mr. Trump and suspected by Mr. Barr. Moreover, a monthslong review by The New York Times found that the main thrust of the Durham inquiry was marked by some of the very same flaws — including a strained justification for opening it and its role in fueling partisan conspiracy theories that would never be charged in court — that Trump allies claim characterized the Russia investigation.

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