Transcript for Trump revokes security clearance for former CIA director

And we begin tonight with the president taking aim and taking away top security clearance from a former top intelligence official who had been critical of the president. President trump today revoking clearance for former CIA director John Brennan, accusing him of erratic conduct and behavior. Brennan responding on Twitter, accusing the president tonight of trying to, quote, punish critics, saying it should, quote, gravely worry all Americans about the cost of speaking out. And tonight, the list of others from the white house press secretary late today that came with a warning that you could be next. ABC's chief white house correspondent Jonathan Karl leading us off. Reporter: Taking aim at one of his harshest critics, president trump today revoked the security clearance of former CIA director John Brennan, denying him access to classified information. In a statement read by press secretary Sarah Sanders, the president invoked Brennan's, quote, wild outbursts on the internet and erratic conduct. Mr. Brennan's line of recent conduct is wholly inconsistent with access to the nation's most closely held secrets. Reporter: In a TV appearance just last night, Brennan lashed out at the president. He is, I think, the most divisive president we've ever had in the oval office. He is feeding and fueling hatredanimosity. Reporter: Brennan is a life-long intelligence official who served under Republican and democratic presidents. He's become a strident trump critic. Even branding as treasonous the president's news conference with Vladimir Putin, where trump said Putin gave an extremely strong and powerful denial of meddling in the 2016 election. It is a betrayal of the nation. He is giving aid and comfort to the enemy. Reporter: Tonight, Brennan tweeted his response to the president's action, calling it "Part of a broader effort by Mr. Trump to suppress freedom of speech and punish critics. It should bravely worry all Americans." And it's not just Brennan in the president's cross hairs. Today, Sanders read a long list of other trump critics who could lose their security clearances, too. James clapper, James Comey, Michael Hayden, Sally Yates, Susan rice, Andrew Mccabe, Peter strzok, Lisa page and Bruce Orr. Reporter: It seems like everybody you mentioned has been a political critic of the president. Is he going after his political opponents with this? No, if there were others that weren't, that we deemed necessary, we would certainly take a look and review those, as well. All right, so, let's get to Jonathan Karl at the white house tonight. John Brennan wasn't the only name mentioned, you just had that list there provided by Sarah Sanders. I want to show viewers at home, here are some of the faces. One thing they have in common, as you pointed out there, Jon, have been critical of decisions made by the president. So, bottom line, just how unusual is it for the president to revoke the security clearance of a former CIA director and warn so many others? Reporter: Well, David, there's no doubt he has the power to do this, but I cannot find another example of a president personally intervening to strip a former official of his security clearance, let alone one who has not been accused of breaking the law. For his part, Brennan says that this is going to have a chilling effect on former officials who may want to come out and question the president's actions. But I have never seen it happen before. Jon Karl leading us off tonight. Jon, thank you.

This transcript has been automatically generated and may not be 100% accurate.