Congressional Democrats have announced the first public hearings next week in an inquiry that may seek to remove President Donald Trump from office.
Three state department officials will testify first. So far lawmakers from three key House committees have heard from witnesses behind closed doors.
The impeachment inquiry centres on claims that Mr Trump pressured Ukraine to publicly announce an investigation into political rival Joe Biden.
US House Intelligence Committee chairman Adam Schiff said the hearings “will be an opportunity for the American people to evaluate the witnesses for themselves, to make their own determinations about the credibility of the witnesses, but also learn first-hand about the facts of the president’s misconduct”
Mr Trump denies any abuse of power.