Former Assistant U.S. Attorney Andy McCarthy reacted Friday to the latest news that U.S. Attorney John Durham's ongoing review into the origins of the Russia investigation has transitioned into a full-fledged criminal probe.

McCarthy -- author of "Ball of Collusion" -- appeared on "Tucker Carlson Tonight" and told guest host Mark Steyn that it's possible former CIA Director John Brennan and former Director of National Intelligence James Clapper may become trial witnesses.

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"I think you might see them as his trial witnesses if there ever ends up being a trial here," McCarthy said. "Let's not forget, Mark, they were called as witnesses by Congress in various proceedings that were relevant to this.

"And in fact, a lot of what I quote from Brennan in the book comes from his own testimony. This is not like, you know, 'Andy figures out what the CIA was up to.' We had this guy in, testifying. And he also, you know, whatever. What he didn't say in his testimony, he said on MSNBC. So, you know, I would think that they're very relevant witnesses."

Fox News reported Tuesday that Durham's probe had expanded significantly based on new evidence uncovered during a recent trip to Rome with Attorney General Bill Barr.

Steyn asked McCarthy if it's likely the CIA is "on the hook" for the alleged misconduct.

"I think that when you look at this as the full continuum of the investigation, the FBI and the Justice Department are almost late to the party. This actually, I think, started as strands of foreign intelligence that came into the government through the CIA and that ended up with the bureau because the CIA acted as kind of a clearinghouse for it," McCarthy said.

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"We coordinate with the foreign intelligence and the foreign government in those places. So there was a lot of foreign activity here," McCarthy added. "That's where I think the real collusion was."

Fox News' Brooke Singman, Gregg Re and Ed Henry contributed to this report.