WASHINGTON – Attorney General William Barr tapped Connecticut's chief federal prosecutor, John Durham, to assist in an investigation into the origins of the Russia inquiry and the FBI's surveillance activities, a person familiar with the matter said Monday.

Durham, a mob-busting federal prosecutor for more than three decades, has assisted the attorney general for several weeks to determine whether federal investigators acted appropriately in the early stages of the now-completed inquiry into Russia's interference in the 2016 election.

Barr announced that he had launched the review last month during an appearance before a Senate subcommittee. He expressed concern about the FBI's use of surveillance involving associates of Donald Trump during the presidential campaign as authorities sought to understand Russia's interference efforts. Barr said he did not know whether officials had done anything wrong.

"Spying on a campaign is a big deal," Barr told lawmakers. "I think spying did occur. The question is whether it was adequately predicated."

The attorney general said he planned to examine the "genesis and the conduct" of the FBI's investigation into possible ties between the Trump campaign and Russia.

"I am not saying that improper surveillance occurred," Barr told the Senate Appropriations subcommittee. "I am concerned about it. There is a basis for my concern."

Durham has led several high-profile special investigations, including an examination of the FBI's handling of criminal informants in Boston during the Clinton administration, which led to the prosecution of former agent John Connolly. He led an inquiry during the George W. Bush administration into the CIA's destruction of videotapes depicting the torture of terror detainees.

“It’s inconceivable to me that the appointment of someone with John Durham’s record would be considered unless the use of a grand jury was contemplated,” said former Attorney General Michael Mukasey, who tapped Durham for the CIA review.

President Trump enthusiastically endorsed Barr's action.

"I am so proud of our attorney general that he is looking into it," Trump said Tuesday.

Democrats seized on Barr's use of the term "spying," asserting that the attorney general sided with Trump to disparage the 22-month investigation, which the president repeatedly described as a "witch hunt."

Last week, FBI Director Christopher Wray said he was unaware of any evidence indicating the FBI abused its surveillance authority, distancing himself from the attorney general. "That's not the term I would use," Wray told the same Senate committee, referring to the "spying" reference.

Rod Rosenstein, until recently the Justice Department's second-in-command, said in a speech Monday that based on what he knew in 2017, "the investigation of Russian election interference was justified, and closing it was not an option."

The review involving the attorney general and Durham marks the third such inquiry into aspects of the Russia investigation led by special counsel Robert Mueller. It was first reported late Monday by The New York Times.

The Justice Department's inspector general is conducting a review of surveillance warrants authorities used to eavesdrop on a former campaign aide, Carter Page, in October 2016. Barr said that effort should be completed by late May or June. The chief federal prosecutor in Utah, John Huber, is in the midst of a separate review.

Trump and Republicans in Congress have complained repeatedly that the FBI targeted the president's campaign for political reasons, revealing text messages between two senior officials involved in the inquiry who expressed their personal contempt for Trump. Critics focused on the FBI's reliance on information from a former British spy who had been hired indirectly by Hillary Clinton's presidential campaign to conduct research on Trump before the election.

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