U.S. Attorney John Durham has already used a grand jury in Connecticut, attorney Joe diGenova said on Tuesday.

It was revealed this week that Attorney General William Barr tasked Durham with examining the origins of the federal Russia investigation.

In a Fox News interview, diGenova said Durham, who is a U.S. attorney in Connecticut, has been working for a couple of months.

"Durham has already used a grand jury in Connecticut, they've already gotten documents, he's already talked to the intel people," he said.

Durham's effort, described by one New York Times source as currently being a review and not a criminal inquiry, is operating alongside the FISA abuse investigation by Justice Department Inspector General Michael Horowitz.

DiGenova said Horowitz has already determined that the three FISA extensions against onetime Trump campaign aide Carter Page were illegally obtained and he is now the brink of finding the first FISA was completely illegal.

Horowitz can recommend that people be prosecuted, and diGenova suggested former FBI Director James Comey and former CIA Director John Brennan have the most to fear.

"This is now big time. This is where Brennan needs five lawyers. Comey needs five lawyers," diGenova said. He also said Brennan will probably see prison time.

DiGenova served as an independent counsel in the 1990s for a case on former President Bill Clinton's passport before he was elected. Last year, it was announced diGenova and his wife and legal partner Victoria Toensing were joining Trump's legal team for the federal Russia investigation, but that plan was nixed within days.

[Read more: Trey Gowdy says John Brennan in more dossier peril than James Comey]