Special Counsel John Durham’s investigation is largely focused on the activities of the FBI as he reviews the origins of the Trump-Russia probe, Fox News has learned, but sources familiar with the investigation said the prosecution of high-level FBI officials, like former Director James Comey, is "unlikely."

Sources told Fox News that the investigation is ongoing and Durham completed the CIA portion of his investigation last year -- and has now shifted his focus to activities of the FBI. 

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One source also told Fox News that Durham had been pursuing "new and credible leads" through the end of the Trump administration, though it is unclear, at this point, what those lines of inquiry entail.

A spokesperson for the special counsel told Fox News that they had "no comment from Mr. Durham." 

Durham’s investigation has produced one criminal charge so far, against former FBI lawyer Kevin Clinesmith, who was accused of altering an email related to the surveillance of former Trump campaign aide Carter Page. That prosecution, though, did not allege a broader conspiracy within the FBI, and the conduct it involved had largely been laid out in a Justice Department inspector general report from last December.

Clinesmith in August pleaded guilty to making a false statement to Durham’s team. The offense carried a maximum term of imprisonment of five years and a fine of up to $250,000.

Last week, Clinesmith was sentenced to 12 months probation and 400 hours of community service, and no fine.

The Justice Department relied on Clinesmith’s assertion as it submitted a third and final renewal application in 2017 to surveil Page under the Foreign Intelligence Surveillance Act.

Durham’s investigation began in May 2019, shortly after special counsel Robert Mueller completed his years-long investigation into whether the Trump campaign conspired with the Russians to influence the 2016 presidential election.

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Mueller’s investigation yielded no evidence of criminal conspiracy or coordination between the Trump campaign and Russian officials during the 2016 election.

Meanwhile, in December, Durham expanded his team, adding new prosecutors, after former Attorney General Bill Barr appointed him in October as special counsel to ensure he could continue his investigation during the Biden Administration.

In the scope order, obtained by Fox News, Barr stated that Durham "is authorized to investigate whether any federal official, employee, or any other person or entity violated the law in connection with the intelligence, counter-intelligence, or law-enforcement activities directed at the 2016 presidential campaigns, individuals associated with those campaigns, and individuals associated with the administration of President Donald J. Trump, including but not limited to Crossfire Hurricane and the investigation of Special Counsel Robert S. Mueller, III."

Under U.S. code, the special counsel would produce a "confidential report" and is ordered to "submit to the Attorney General a final report, and such interim reports as he deems appropriate in a form that will permit public dissemination."

As for Comey, the former FBI director last year said Durham's probe was not concerning for him.

"I have had no contact with him and haven’t talked to him," Comey told CBS News' "Face the Nation" in August. "I can’t imagine that I’m a target."

A spokesman for Comey did not immediately respond to Fox News' request for coment. 

Also over the summer, Durham's team questioned former CIA Director John Brennan for some eight hours at CIA headquarters. Brennan later said through a spokesman he was assured he was "not a target."

Brennan, in December, during an appearance on "Fox News Sunday," said he had no problem with Durham's investigation continuing into 2021. 

"I think that is fine, I have no problems with it," Brennan said, adding that he has already sat down with Durham's team for eight hours. "I do believe that John Durham is going to carry out his responsibilities ably and hopefully not with any political influence."