National Security Adviser John Bolton suggested on Sunday that there may be a classified smoking gun that proves collusion between the Trump campaign and Russians.

During an interview with ABC that aired on Sunday, Bolton backed Donald Trump's decision to strip former CIA Director John Brennan of his security clearance. Brennan has been a frequent critic of Trump.

"A number of people have commented that [John Brennan] couldn't be in the position he's in, of criticizing President Trump and his so-called collusion with Russia, unless he did use classified information," Bolton told ABC.

"But I don't know the specifics," he added, explaining that he had no evidence of Brennan misusing classified information.