since leaving his white house post. cnn's brian todd is on this story. >> he's been one of the ghosts of the impeachment hearings, on a list of key figures like mike pompeo, rudy giuliani and mike mulvaney who didn't testify, but john bolton could know more than all of them about the allegations that president trump used his power to investigate the bidens. >> he was one of the biggest starring players in this whole episode with ukraine and yet he was an off stage character almost the entire time in the impeachment probe. >> but on friday, a tease from john bolton. in a series of tweets, he accused the white house of blocking access to his personal twitter account. >> they attached software to it. and twitter unattached the software to it.