(CNN) The top federal prosecutor in Connecticut is assisting Attorney General Bill Barr in his review of the genesis of the 2016 Russia investigation, according to a source familiar with the review.

US attorney John Durham has been tapped to help Barr, but is not acting as a special counsel like Robert Mueller, the source said, and will be looking at whether intelligence collection activities by the US government prior to election day were lawful. Durham has been working on this for "weeks," the source added.

Durham is a Trump appointee who has investigated numerous public corruption cases under Republican and Democratic administrations. He was appointed by then-Attorney General Janet Reno to investigate corruption surrounding the use of FBI informants in Boston in the late 1990s, and later called on to investigate the CIA's destruction of videotapes of detainee interrogations in 2008.

And while Barr has said publicly that he's not launching a full blown investigation into the FBI's handling of the matter, Durham's role -- first reported by The New York Times -- does provide one of the clearest signals to date of the seriousness of an inquiry President Donald Trump had called for repeatedly.

Barr's concerns about surveillance of the Trump campaign were first revealed during testimony before the House Appropriations Committee last month when he suggested, controversially, that Trump's campaign was spied on , but he has declined to elaborate on the precise contours of the review or the basis of concern.

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