The whistleblower whose complaint has led to an impeachment inquiry against Donald Trump is a CIA officer, US media report.
The New York Times said the unnamed officer once worked at the White House.
The whistleblower says senior White House officials tried to “lock down” all details of a phone call between Mr Trump and the Ukrainian president.
In the call, 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”.