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House Intelligence Committee chairman Adam Schiff is leading the inquiry
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 Mr Biden, who is now a Democratic White House contender.
The US president has been touting unsubstantiated corruption claims about the former US vice-president, whose son, Hunter Biden, worked for a Ukrainian gas company.
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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 Wednesday is career state department official George Kent.
Mr Kent reportedly told lawmakers that state department officials had been sidelined as the White House put political appointees in charge of Ukraine policy.
He also testified that he had been warned to “lay low” by a superior after expressing concern about Mr Trump’s personal lawyer Rudy Giuliani, who was lobbying Ukraine to investigate the Bidens.
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Former US ambassador to Ukraine Marie Yovanovitch, who was recalled in May after falling from favour with the White House, is due to testify on Friday of next week.
She told the hearing last month that she felt threatened by Mr Trump’s remark about her to Ukraine’s president that “she’s going to go through some things”.
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.”