Senior White House officials tried to "lock down" all details of a phone call between Donald Trump and the Ukrainian president, according to a whistleblower complaint against the US president.
In the call, Mr Trump pushed Volodymyr Zelensky to investigate his leading domestic political rival, Joe Biden.
The newly released complaint says the call transcript was not stored in the usual computer system.
Instead it was stored in a separate system used for classified information.
Nancy Pelosi, the most senior Democrat, announced on Tuesday that the party was pushing ahead with a formal impeachment inquiry against the Republican president.
She accused Mr Trump of seeking foreign help in the hope of smearing Mr Biden - who is seeking the Democratic nomination for the 2020 presidential election - and of using military aid to Ukraine as a bargaining tool.
Mr Trump acknowledged that he personally blocked nearly $400m in military aid to Ukraine days before he spoke to Mr Zelensky, but denied that it was to pressure the Ukrainian leader into investigating Mr Biden.
The release of the whistleblower complaint came as US lawmakers on the House of Representatives intelligence committee began to question President Trump's top intelligence official on the issue. Acting National Intelligence Director Joseph Maguire had initially refused to share the complaint with Congress.
But under questioning by the House committee on Thursday, Mr Maguire said he believed the whistleblower had acted in "good faith" and "did the right thing".