The review into the origins of the Russia investigation is growing as federal prosecutors are reportedly looking to question current and former intelligence officials.

The investigation launched by Attorney General William Barr and currently being conducted by Connecticut U.S. Attorney John Durham is reportedly expanding and could soon include interviews of former CIA Director John Brennan and former director of national intelligence James Clapper, Brennan told NBC News.

About two dozen former and current FBI officials have also been interviewed by prosecutors, with two former senior FBI agents assisting in the probe, according to The New York Times, which noted that the “number of interviews shows that Mr. Durham’s review is further along than previously known.”

During a press briefing on Thursday, acting White House chief of staff Mick Mulvaney called the Durham probe “an ongoing investigation by our Department of Justice into the 2016 election.“

While a notable number of FBI officials have been interviewed, key figures involved at the beginning of the Russia investigation remain to be questioned in Durham’s review, including former counterintelligence agent Peter Strzok who opened the inquiry, the bureau’s general counsel James Baker, as well as fired FBI director James Comey and his deputy, Andrew McCabe.

According to NBC News:

Durham has also requested to talk to CIA analysts involved in the intelligence assessment of Russia’s activities, prompting some of them to hire lawyers, according to three former CIA officials familiar with the matter. And there is tension between the CIA and the Justice Department over what classified documents Durham can examine, two people familiar with the matter said. With Barr’s approval, Durham has expanded his staff and the timeframe under scrutiny, according to a law enforcement official directly familiar with the matter. And he is now looking into conduct past Donald Trump’s inauguration in January 2017, a Trump administration official said.

It is not clear if Durham is actually conducting a criminal investigation, which is not what it was when it started and the Justice Department has not elaborated on any allegations of wrongdoing, according to NBC News. As it has not been addressed as a criminal investigation, witnesses are voluntarily interviewed as subpoenas cannot be issued for testimony or for documents.

Brennan called the probe “bizarre” and said, “I don’t know what the legal basis for this is.”

The vocal critic of President Trump who is now MSNBC’s senior national security analyst was slammed by the president last year as “a loudmouth, partisan, political hack.”

“Has anyone looked at the mistakes that John Brennan made while serving as CIA Director? He will go down as easily the WORST in history & since getting out, he has become nothing less than a loudmouth, partisan, political hack who cannot be trusted with the secrets to our country!” the president tweeted.

Conservative radio host Rush Limbaugh revealed earlier this month that Obama’s CIA director had “traveled to Ukraine back in the time period around 2016 under a fake passport so it wouldn’t be known that he had gone, and he was arranging data on the dossier and all of the other dirt that they were trying to amass.”