The whistleblower whose complaint has led to an impeachment inquiry against President Donald Trump is a CIA officer, US media report.
The unnamed officer once worked at the White House, several US outlets said.
The whistleblower says senior White House officials tried to "lock down" all details of a phone call between Mr Trump and the Ukrainian president.
Mr Trump demanded to know who gave information to the whistleblower, saying the source was "close to a spy".
In the call on 25 July, Mr Trump pushed Volodymyr Zelensky to investigate his leading Democratic political rival, Joe Biden.
The complaint, released on Thursday, says the call transcript was not stored in the usual computer system. Instead it was stored in a separate system used for classified information.
Impeachment is a two-stage political process, rarely exercised, by which a US president can be removed from office for wrongdoing.
Even if President Trump was impeached by the House of Representatives, he is unlikely to be forced out of the White House because Republicans control the Senate.
Speaking to BBC's US affiliate CBS News, 2016 Democratic White House candidate Hillary Clinton said she supported an impeachment inquiry into the "illegitimate president", whom she also called a "corrupt human tornado".
Mr Trump's re-election campaign has enjoyed a fundraising bump since the Democratic impeachment inquiry was announced on Tuesday. It has raised $13m (£10.5m), campaign officials told AP News.
But the Ukraine controversy has also spawned a surge in donations to Mr Biden. His aides said the former vice-president has had his best week of fundraising since his campaign launched.
Why is the phone call controversial?
A rough transcript of the conversation, released by the White House this week, shows that the president urged his newly elected Ukrainian counterpart to investigate discredited corruption allegations against Mr Biden and his son Hunter.
"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that," he told Mr Zelensky.
Democrats accuse Mr Trump of illegally seeking foreign help in the hope of smearing Mr Biden - who is seeking the Democratic nomination for the 2020 presidential election.
The US president also called on the Ukrainian leader to talk to US Attorney General William Barr and Mr Trump's lawyer Rudy Giuliani about investigating Hunter Biden's past business dealings in Ukraine.
Mr Trump is also accused of using military aid to Ukraine as a bargaining tool. The package - which has since been released - had been appropriated by Congress to support the interests of US interests in a friendly country.
Mr Trump, a Republican, denies any wrongdoing and has dismissed the impeachment proceedings as a "hoax" and "another witch-hunt".
He acknowledged that he had personally blocked nearly $400m (£324m) 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.
Who is the whistleblower?
Few details are known. A lawyer for the whistleblower warned that trying to identify the person could place them "in harm's way".
The New York Times, Washington Post, and Reuters news agency identified the whistleblower as a CIA officer.
Meanwhile, an audio recording has emerged in which Mr Trump demands to know who provided information to the whistleblower - and describes them as "close to a spy".
"I want to know who's the person, who's the person who gave the whistleblower the information. Because that's close to a spy," the president said in private remarks to staff at the UN. The recording was provided to the Los Angeles Times.
In an apparent reference to the execution of spies by the US in the past, he adds: "You know what we used to do in the old days when we were smart? Right?"
The remarks were condemned by the Democratic chairmen of three House committees in the House of Representatives. In a joint statement they said the comments constituted "reprehensible witness intimidation".
One Democrat said he wanted the whistleblower to talk to members of the House Intelligence Committee "at the earliest".
"I am concerned at some of the statements the president has been making about the whistleblower, and whether he's going to retaliate against the guy," said Representative Raja Kirshnamoorthi.
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 declassified document characterises the president's conduct as a "serious or flagrant problem, abuse, or violation of law".
The whistleblower says 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 had been stored in a "stand-alone computer system reserved for codeword-level intelligence information, such as covert action".
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.
What's happened in Congress?
The release of the whistleblower complaint on Thursday came as lawmakers on the House 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 committee on Thursday, Mr Maguire said he believed the whistleblower had acted in "good faith" and "did the right thing".
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."
The president hit back at Mr Schiff on Friday for reading what the lawmaker later described as a "parody" version of the phone call, depicting Mr Trump as a mafia boss.
...sound horrible, and me sound guilty. HE WAS DESPERATE AND HE GOT CAUGHT. Adam Schiff therefore lied to Congress and attempted to defraud the American Public. He has been doing this for two years. I am calling for him to immediately resign from Congress based on this fraud!— Donald J. Trump (@realDonaldTrump) September 27, 2019
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."
What is the claim about Joe Biden?
During the call with Mr Zelensky, Mr Trump raised the 2016 removal of a Ukrainian prosecutor, Viktor Shokin.
He went on to discuss Hunter Biden and the unsubstantiated allegation that his father - then the US vice-president - stopped an investigation of his son by lobbying Ukraine to fire Mr Shokin.
The Ukrainian prosecutor general's office had an open inquiry into Burisma, a natural gas company on which Hunter Biden was a board member.
No evidence has emerged 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 US justice department said on Wednesday that Mr Trump had not spoken to the attorney general about having Ukraine investigate Mr Biden.
Speaking at a fundraising event in San Marino, California, on Thursday, Mr Biden said: "It's a tactic that's used by this president to try to hijack an election so we do not focus on the issues that matter in our lives."
How the controversy unfolded
- Mid-July - President Trump orders White House aide to hold back almost $400m in military aid to Ukraine, report US media
- 25 July - Mr 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.