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 in the televised hearings, which have until now been held 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.
Three key House committees have already heard private testimony from more than two dozen witnesses.
House Intelligence Committee chairman Adam Schiff, who is overseeing 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.”
The Capitol Hill hearings will now be broadcast live, with both Democratic and Republican lawmakers questioning witnesses.
The first public witness next week will be Bill Taylor, acting US ambassador to Ukraine, who delivered some of the most explosive private testimony last month.
On Wednesday, House Democrats released a transcript of his evidence.
It shows Mr Taylor told lawmakers he understood the president had withheld nearly $400m (£310m) in US military aid because he wanted Ukraine to investigate the Bidens.
Mr Trump has been making unsubstantiated corruption claims about former US vice-president Mr Biden, whose son, Hunter Biden, worked for a Ukrainian gas company.
Joe Biden is a Democratic White House contender for the presidential election a year from now.
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 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.
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 Democrats formally launched the impeachment inquiry after an intelligence official filed a whistleblower complaint in September.
The whistleblower raised the alarm about a 25 July phone call in which Mr Trump asked Ukraine’s president to investigate the Bidens.
Source: BBC