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 withheld aid to Ukraine to prod it to investigate his political rival, Joe Biden.
Mr Trump denies any abuse of power.
The first public witness will be Bill Taylor, acting US ambassador to Ukraine, who delivered some of the most explosive private testimony last month.
On Wednesday – a week ahead of his scheduled public hearing – House Democrats released a transcript of his evidence.
It shows Mr Taylor told lawmakers it was his “clear understanding” that the president had withheld nearly $400m (£310m) in US military aid because he wanted Ukraine to investigate the Bidens.
Mr Trump has been making discredited corruption claims about former US vice-president Mr Biden, whose son, Hunter Biden, worked for a Ukrainian gas company.
Joe Biden is a Democratic front-runner for the presidential election a year from now.