Former New Jersey Gov. Chris Christie said Donald Trump should be very worried about a close member of his inner circle from his final year in office.

“I’ve said all along I think Mark Meadows is already a cooperating witness,” Christie told CNN’s Anderson Cooper on Monday. “He has all the looks of a cooperating witness, running into coffee shops away from the press.”

Meadows was White House chief of staff during much of the final year of Trump’s presidency including the period after the election, which is the focus of the indictment on federal charges of election obstruction and conspiracy.

Christie, who was also a member of the Trump inner circle during that period, said the former president should have special reason to be concerned about Meadows’ potential cooperation.

“He was a very involved chief of staff, in my experience,” Christie said. “He made sure he was in every meeting and every conversation. And we remember, there are hundreds of text messages that he turned over to the special counsel that he kept.”

Christie said that Meadows could deliver the “worst testimony” against Trump of anyone other than family members.

A number of observers have zeroed in on Meadows as someone who may be cooperating with prosecutors given his months of near silence. That belief was only bolstered by the indictment, where Meadows is barely mentioned despite his close involvement in White House activity in the period between the 2020 election and the Jan. 6 attack on the U.S. Capitol.

Christie was one of the first prominent Republicans to endorse Trump in 2016 after abandoning his own presidential campaign, and was close to him through much of his presidency ― even developing a COVID-19 infection while helping to prep Trump for the 2020 debates.

He spent a week in the ICU.

Christie has since turned into a Trump critic and is running against him for the Republican presidential nomination.

See more of Christie’s conversation with Cooper below: