President Donald Trump speaks to reporters after arriving aboard Air Force One at Joint Base Andrews, Maryland, September 26, 2019. Jonathan Ernst | Reuters

President Donald Trump on Thursday raged about a whistleblower's bombshell complaint at a fundraising event, comparing its sources to spies and suggesting they committed "treason," several reports said. Trump fumed about the whistleblower, who is a member of the intelligence community, as the explosive complaint about the president's actions became public Thursday morning. The complaint alleges that Trump has been engaged in an effort to get Ukraine to investigate a leading political opponent, former Vice President Joe Biden, and that White House officials took steps to cover up evidence of that effort.

"I want to know who's the person, who's the person who gave the whistle-blower the information? Because that's close to a spy," Trump said at a private breakfast for staff from the United States Mission in New York, according to the Los Angeles Times.

"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," Trump said, according to the LA Times. The New York Times, which published a similar account of Trump's remarks, reported that the president's comment "stunned people in the audience." The New York Times article cited a person briefed on what took place, who had notes of what Trump said. Diplomats attending the event were taken aback to hear such explicitly political comments and attacks on the president's perceived enemies during an event for government workers, an official told NBC News The crowd at the breakfast included U.S. Ambassador to the United Nations Kelly Craft.

Photo illustration on September 26, 2019 shows redacted pages of the whistleblower complaint referring to President Donald Trump's call with his Ukrainian counterpart Volodymyr Zelensky. Eva Hambach | AFP | Getty Images