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, but 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.
President Trump has dismissed the impeachment proceedings as a "hoax" and a "witch-hunt", and has been tweeting after the complaint was publicly released.
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".
What does the complaint say?
The whistleblower's complaint accuses Mr Trump of "using the power of his office to solicit interference from a foreign country in the US 2020 election".
The now unclassified document characterises the president's conduct as a "serious or flagrant problem, abuse, or violation of law".
The alleged violation concerns President Trump asking his Ukrainian counterpart on 25 July this year to "do me a favour" by investigating unfounded allegations against Mr Biden.
The whistleblower says in the complaint that they had learned from several sources that senior White House officials had intervened to "lock down" all records of the call, particularly an official word-for-word transcript.
"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the whistleblower wrote in the complaint.
The whistleblower says details of the call were stored in a "stand-alone computer system reserved for codeword-level intelligence information, such as covert action".
The whistleblower adds that officials said it was "'not the first time' under this administration that a presidential transcript was placed into this codeword-level system solely for the purpose of protecting politically sensitive - rather than national security sensitive - information".
The complaint makes clear that the whistleblower was "not a direct witness" to most of the events described, but found their colleagues' accounts to be credible "because, in almost all cases, multiple officials recounted fact patterns that were consistent with one another".
What's happening in Congress?
The House intelligence committee chairman Adam Schiff, a Democrat, opened the hearing by accusing President Trump of a "classic organised crime shakedown".
But the leading Republican on the committee, Devin Nunes, a Trump supporter, said: "I want to congratulate the Democrats on their latest informational warfare operation against the president and their extraordinary ability to once again enlist the mainstream media in their campaign."
Mr Schiff asked Mr Maguire why he had sought advice from the White House before deciding to release the whistleblower's report.
"It just seemed prudent to check," Mr Maguire responded, saying he had sought the advice of White House counsel to determine if the report included information protected by presidential executive privilege.
He added: "I believe everything in this matter is totally unprecedented."
During further questioning by Mr Schiff, Mr Maguire said he believed the whistleblower had acted in "good faith".
He continued: "I think the whistleblower did the right thing. I think he followed the law every step of the way."
What is the claim about Joe Biden?
During the call, Mr Trump discussed with newly elected Mr Zelensky the 2016 removal of a Ukrainian prosecutor, Viktor Shokin.
He then went on to discuss Mr Biden's son, Hunter Biden, and the unsubstantiated allegation that Mr Biden - then the US vice-president - stopped the prosecution of his son by lobbying Ukraine to fire Mr Shokin.
Mr Shokin's office had opened an investigation into Burisma, a natural gas company on which Hunter Biden was a board member.
During the call, Mr Trump also asked Mr Zelensky to work with US Attorney General William Barr and Mr Trump's personal lawyer, Rudolph Giuliani, to look into the matter.
There is no evidence of any wrongdoing by the Bidens.
Mr Biden, along with other Western officials, had called for Mr Shokin to be fired because of the perception that he was soft on corruption.
When Mr Shokin was replaced, his successor continued to investigate Burisma for 10 months before the inquiry ended.
The Department of Justice said on Wednesday that Mr Trump had not spoken to the attorney general about having Ukraine investigate Mr Biden, and Mr Barr had not communicated with Ukraine.
How the controversy unfolded:
- 18 July - President Trump orders White House aide to hold back almost $400m in military aid to Ukraine, report US media
- 25 July - President Trump speaks to Ukraine's leader in a 30-minute phone call
- 9 September - Congress learns of a whistleblower's complaint about the call, but is blocked by the Trump administration from viewing it
- 11 September - Military aid for Ukraine is cleared for release by the Pentagon and Department of State
- 23 September - Mr Trump confirms he withheld Ukrainian aid, saying it was due to concerns about "corruption"
- 24 September - Mr Trump says the aid was withheld so that other countries would pay more.