Congressional Democrats have announced the first public hearings next week in their inquiry that could seek to remove President Donald Trump from office.
Three state department officials will testify first in the televised hearings which have previously been held behind closed doors.
The inquiry centres on allegations that Mr Trump's administration withheld aid to Ukraine to prod it to investigate his political rival, Joe Biden.
Mr Trump denies any wrongdoing.
Three key House committees have already heard private testimony from more than two dozen witnesses.
The first public witness next week will be Bill Taylor, acting US ambassador to Ukraine.
He has already told the inquiry that Mr Trump, a Republican, wanted Ukraine to investigate former US Vice-President Joe Biden, who is now a Democratic White House contender.
Mr Taylor spoke of an "irregular, informal channel of US policy-making" that had "fundamentally undermined" relations with Ukraine.
Also scheduled to testify publicly next week are former US ambassador to Ukraine Marie Yovanovitch and career state department official George Kent.
House Intelligence Committee chairman Adam Schiff, who is leading the inquiry, told reporters on Wednesday that an impeachment case was building against the president.
He said: "We are getting an increasing appreciation for just what took place during the course of the last year.
"And the degree to which the President enlisted whole departments of government in the illicit aim to get Ukraine to dig up dirt on a political opponent."