A 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 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 Zelensky, 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 W. 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 a U.S. intelligence official, who previously worked in the White House, had lodged a complaint with the inspector general for the intelligence community. 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.

He and the members spent much of their time discussing the process Atkinson followed, the statute governing his investigation of the complaint and the nature of an “urgent concern” that he believed it represented, according to a person familiar with the briefing, who, like others, spoke on the condition of anonymity.

“He was being excruciatingly careful about the language he used,” the person said.

Atkinson made clear that he disagreed with a lawyer for the Office of the Director of National Intelligence, who had contradicted the inspector general and found that the whistleblower complaint did not meet the statutory definition of an urgent concern because it involved a matter not under the DNI’s jurisdiction.

Atkinson told lawmakers he disagreed with the lawyer’s analysis — meaning he felt the matter was under the DNI’s purview — and also that it was urgent “in the common understanding of the word,” the person said.

Following the meeting, House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) warned of possible legal action if intelligence officials did not share the whistleblower complaint.

Schiff called acting director of national intelligence Joseph Maguire’s refusal to share the complaint with Congress as “unprecedented” and said he understood the Justice Department was involved in that decision.

“We cannot get an answer to the question about whether the White House is also involved in preventing this information from coming to Congress,” Schiff said, adding: “We’re determined to do everything we can to determine what this urgent concern is to make sure that the national security is protected.”

Someone, Schiff said, “is trying to manipulate the system to keep information about an urgent matter from the Congress … There certainly are a lot of indications that it was someone at a higher pay grade than the director of national intelligence.”

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 a Sept. 17 letter to intelligence committee leaders, Atkinson wrote that he and Maguire “are at an impasse” over how the whistleblower could contact the congressional committees. Ordinarily, a matter of urgent concern that the inspector general deems credible is supposed to be forwarded to the intelligence oversight panels in the House and Senate.

But Maguire prevented Atkinson from doing so, according to correspondence that has been made public. Atkinson wrote that he had requested permission from Maguire to inform the congressional intelligence committees about the general subject matter of the complaint, but was denied.

Maguire, Atkinson wrote, had consulted with the Justice Department, which determined that the law didn’t require disclosing the complaint to the committee because it didn’t involve a member of the intelligence community or “an intelligence activity under the DNI’s supervision.”

Atkinson faulted the Justice Department’s conclusion “particularly … and the Acting DNI’s apparent agreement with the conclusion, that the disclosure in this case does not concern an intelligence activity within the DNI’s authority.”

Maguire is scheduled to testify before the intelligence committee in a public session next Thursday.

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.

“As the 2020 election draws closer, President Trump and his personal attorney appear to have increased pressure on the Ukrainian government and its justice system in service of President Trump’s reelection campaign, and the White House and the State Department may be abetting this scheme,” the chairmen of the House Intelligence, Foreign Affairs and Oversight committees wrote, citing media reports that Trump had threatened to withhold $250 million in aid to help Ukraine in its ongoing struggle against Russian-backed separatists.

Lawmakers also became aware in August that the Trump administration may be trying to stop the aid from reaching Ukraine, according to a congressional official.

Giuliani, Trump’s personal lawyer, dismissed the reports of the whistle blower and Trump’s “promise” to a foreign leader.

“I’m not even aware of the fact that he had such a phone call,” Giuliani said Thursday. “If I’m not worried about it, he’s not worried about it.”

House Democrats are looking into whether Giuliani traveled to Ukraine to pressure that government outside of formal diplomatic channels to effectively help the Trump reelection effort by investigating Hunter Biden about his time on the board of Burisma, a Ukrainian gas company.

The filing of the whistleblower complaint has led to what veterans of U.S. spy agencies described as an unprecedented situation with potentially grave consequences for the already troubled relationship between the president and the nation’s powerful intelligence community.

It remains unclear how the whistleblower gained access to details of the president’s calls — whether through so-called “readouts” generated by White House aides, or other means.

Memos that serve as transcripts of such calls are created routinely. But if that is the source in this instance, it would appear to mean that White House aides made a formal record of comments by the president later deemed deeply troubling by the intelligence community’s chief watchdog.

John Wagner, Karoun Demirjian, Robert Costa and Josh Dawsey contributed reporting.