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.