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.
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.
He 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 (£322m) 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.
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.
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.
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”.
