Brennan has been among the president's most vocal critics. Mullen said on Sunday that he does not support the former CIA director being as critical of Trump as he has been but believes that Brennan and other former officials should not have to fear that they will be stripped of their security clearances because of their criticism. "I am concerned about the whole issue of free speech. And as long as John is not revealing classified information that he shouldn't, then I certainly think he has a right to speak," Mullen said. Earlier, 14 former CIA directors and deputy directors from Republican and Democratic administrations, as well as a former director of national intelligence, called Trump's revocation of Brennan's clearance a blatant attempt to "stifle free speech" and send an "inappropriate and deeply regrettable" signal to other public servants. As criticism of Trump's move has intensified, the president has showed no signs of backing down. According to senior administration officials, the White House has drafted documents revoking the clearances of other current and former officials who Trump has demanded be punished for criticising him or playing a role in the investigation of Russian interference in the 2016 election. Others on Sunday morning rallied to Trump's defense.

National security adviser John Bolton backed the idea of a formal review to determine whether former officials should keep their security clearance and said Brennan may have "crossed the line." "I think a number of people have commented that he couldn't be in the position he's in of criticising President Trump and his so-called collusion with Russia unless he did use classified information," Bolton said in an interview on ABC News's This Week, although he added that he has no knowledge of a single, specific instance of that. "What I do know was when he was director of CIA, I was very troubled by his conduct, by statements he made in public and by what I thought was his politicisation of the intelligence community," the national security adviser added. Bolton said that he did not consider a political disagreement sufficient grounds for an official to have a security clearance pulled but that violating the separation between intelligence and policy would be grounds. He said that "senior intelligence officials, career intelligence officials who come out of the government" should "keep that wall of separation" between intelligence and policy, and that Brennan had not done that." Bolton later added, "There is a line, and somebody can cross it."

Bolton said it is time to review policies on security clearances for former US officials. "I think it's certainly appropriate, in a time when we're seeing what I believe are unprecedented leaks of highly classified information, to look at the question of how many people have clearances, how many people received this very sensitive information, both inside the government and in the case of former officials," he said. "So, I don't see that there would be anything wrong if it were determined to go that way to review the policies about former officials having clearances." The Washington Post