WASHINGTON, Sept. 20 - A U.S. whistleblower complaint about President Trump made by an intelligence official centers on Ukraine, according to two people familiar with the matter, which has set off a struggle between Congress and the executive branch, the Washington Post reported Friday.

The complaint involved communications with a foreign leader and a “promise” that Trump made, which was so alarming that a U.S. intelligence official who had worked at the White House went to the inspector general of the intelligence community, two former U.S. officials said.

Two and a half weeks before the complaint was filed, Trump spoke with Ukrainian President Volodymyr Zelenskiy, a comedian and political newcomer who was elected in a landslide in May.

That call is already under investigation by House Democrats who are examining whether Trump and his attorney Rudolph Giuliani sought to manipulate the Ukrainian government into helping Trump’s reelection campaign. Lawmakers have demanded a full transcript and a list of participants on the call.

A White House spokesperson declined to comment.

The Democrats’ investigation was launched earlier this month, before revelations that an intelligence official had lodged a complaint with the inspector general. The Washington Post first reported on Wednesday that the complaint had to do with a “promise” that Trump made when communicating with a foreign leader.

On Thursday, the inspector general testified behind closed doors to members of the House Intelligence Committee about the whistleblower’s complaint.

Over the course of three hours, Michael Atkinson repeatedly declined to discuss with members the content of the complaint, saying he was not authorized to do so.

Trump has denied doing anything improper. In a tweet Thursday morning, the president wrote, “Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself.

“Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially ‘heavily populated’ call,” Trump wrote.

In letters to the White House and State Department, top Democrats earlier this month demanded records related to what they say are Trump and Giuliani’s efforts “to coerce the Ukrainian government into pursuing two politically-motivated investigations under the guise of anti-corruption activity” — one to help Trump’s former campaign chairman Paul Manafort, who is in prison for illegal lobbying and financial fraud, and a second to target the son of former vice president Joe Biden, who is seeking the Democratic nomination to challenge Trump. (wp/ez)