Lead House impeachment investigator Adam Schiff says he will release transcripts of the closed-door interviews US lawmakers have conducted with current and former Trump administration officials.
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Democrat Schiff says in a letter to colleagues that investigators will unveil the transcripts when "it will not jeopardise" the investigation into President Donald Trump's alleged political interests in his dealings with Ukraine.
Schiff says he expects to hold public hearings, but isn't saying who the witnesses might be.
Schiff chairs the House Intelligence Committee.
Members from that and other House panels have been questioning officials about Trump's efforts to pressure Ukraine to investigate former Vice President Joe Biden, a potential Democratic presidential candidate.
Republicans attending the private depositions have complained about the sessions' secrecy and claimed Democrats have run them unfairly.
Democrats say secrecy is needed to prevent witnesses from coordinating their stories.
Michael McKinley, a former adviser to US Secretary of State Mike Pompeo spoke to the Democratic-led inquiry on Wednesday.
McKinley resigned abruptly last week and is the latest senior administration official to give closed-door testimony.
He faced questions from investigators as Republicans in Congress stepped up attacks on a process that could end in Trump's impeachment by the House of Representatives before the end of the year.
McKinley said he resigned because he "could no longer look the other way" as colleagues were denied professional support and respect.
The career foreign service officer and top aide said he was "disturbed" by administration efforts to obtain negative information on political opponents.
The testimony is according to a former colleague of McKinley's familiar with the testimony who requested anonymity.
House Democrats' impeachment probe is focused on efforts by Trump to urge Ukraine to investigate political rival Joe Biden and his family.
McKinley said the department failed to support foreign service employees caught up in the Ukraine inquiry.
The Washington Post reported he resigned over State Department leadership's unwillingness to defend former US ambassador to Ukraine Marie Yovanovitch against an effort to intervene in the country for Trump's political benefit.
Yovanovitch, who was withdrawn earlier this year, told investigators last week the president ousted her on "unfounded and false claims".
Previous witnesses have described how policy staff were sidelined on Ukraine by Trump's personal lawyer, Rudy Giuliani, and others.
Trump calls the impeachment inquiry an "illegitimate process".
A House vote to impeach Trump would lead to a trial in the Republican-controlled Senate that could result in his ouster, but so far, few Senate Republicans have criticised the president.
The House probe is focused on a July 25 phone call in which Trump pressed Ukrainian President Volodymyr Zelenskiy to investigate unsubstantiated allegations against Biden and his son, Hunter Biden, who was on the board of a Ukrainian gas company.
Democrats accuse Trump of pressuring Ukraine to dig up dirt on a domestic rival after withholding $US391 million ($A578 million) in US security aid to help combat Russian-backed separatists.
Australian Associated Press