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.
Democrats accuse 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, a Republican, denies any wrongdoing and has dismissed the impeachment proceedings as a “hoax” and “another witch-hunt”.
He acknowledged that he 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 spies and treason, we used to handle it a little differently than we do now…”
The remarks were condemned by the Democratic chairmen of three committees in the House of Representatives. In a joint statement they said the comments constituted “reprehensible witness intimidation” and an attempt to obstruct the impeachment inquiry.
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 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”.