impeachment inquiry, though it's not clear bolton will agree to appear without a subpoena. source of sources also tell cnn that bill taylor is willing to return to capitol hill to testify in public, a particularly monumental moment in the house democrats' intensifying impeachment inquiry. taylor's testimony last week behind closed doors sent shock waves through the capital, where he completely undercut the administration's defense that there was no quid pro quo with ukraine. meantime, two new witnesses testifying on capitol hill today. christopher anderson, aid to former special envoy to ukraine, kurt volcker, speaking to lawmakers behind closed doors, about the concerns voiced by bolton over rudy giuliani's shadow ukraine operation. anderson, according to his opening statement obtained by cnn, saying bolton cautioned mr. giuliani was a key voice with the president on ukraine, which could be an obstacle to