President Donald Trump made a pledge to a foreign leader that a U.S. intelligence official found so concerning, they filed a formal whistleblower complaint about the president, according to a new report.

Citing two former officials familiar with the matter, the Washington Post reported Wednesday that Trump’s “promise” to a foreign leader had prompted the official to file a formal complaint with the inspector general of the intelligence community, Michael Atkinson.

The complaint is at the center of a fierce dispute between Congress and the acting director of national intelligence, Joseph Maguire, who has refused to hand over details about the complaint to the House Intelligence Committee. But after strained negotiations in recent days, Maguire has agreed to testify before the committee on Sept. 26, in what promises to be an explosive confrontation.

With the committee chairman Adam Schiff (D-Calif.) complaining he’s in the dark about the whistleblower’s complaint and pushing for answers, much about the affair remains murky. But here’s what we know so far, and what committee members will be trying to find out.

What we know

According to the Post’s report, the conduct that raised alarm was a promise made by Trump to a foreign leader.

One of the newspaper’s sources, a former U.S. official speaking on condition of anonymity, said the promise was made over a phone call.

The complaint was filed on Aug. 12, when Trump was at his resort in New Jersey.

What we don’t

Who the world leader was. In the five weeks prior to the complaint being filed, Trump spoke to or interacted with five foreign leaders, according to White House records: Vladimir Putin, Kim Jong Un, the Emir of Qatar Tamim bin Hamad Al Thani, Imran Khan of Pakistan, and Dutch Prime Minister Mark Rutte.

It’s not clear which — if any — of these leaders the whistleblower’s complaint related to. But according to the Post, Trump initiated a phone call with Putin on July 31, received two letters from Kim over the summer, and publicly stated in June, in response to media reports that the CIA had recruited Kim’s half-brother, that such a thing would never happen on his watch.

What the promise was. As with the identity of the other leader involved, that remains a mystery for now.

Who the whistleblower is — and who else may be compromised. Schiff has said he doesn’t know the answer to either of these questions, and is demanding to find out. In a letter to Maguire’s office last week he suggested that senior administration officials other than the president could also be involved, writing that from what he’d gleaned, he believed "the serious misconduct at issue involves the President of the United States and/or other senior White House or Administration officials.”

On Monday, he said he still didn’t know exactly what the complaint was about, or who had blown the whistle.

What happens next?

The revelation raises further concerns about Trump’s freewheeling interactions with foreign governments, and his cavalier approach to classified information — which has included jeopardizing a source of intelligence on ISIS in a 2017 White House meeting with the Russian foreign minister. It’s also likely to further strain his testy relationship with the U.S. intelligence establishment, already frayed by scrutiny of his campaign’s dealings with Russia in the 2016 presidential campaign.

Prior to the Post’s report, the whistleblower’s complaint had already triggered a fierce tug-of-war between Congress and the intelligence community over whether details should be provided to lawmakers, and Schiff had suggested Trump could potentially be implicated. But the report was the first tying Trump directly to the scandal.

Schiff has demanded that his committee be fully briefed on the whistleblower’s complaint, but Maguire refused to do so by Tuesday’s deadline, saying the matter was beyond his jurisdiction. In a response to congressional leaders, his office argued that the complaint was not of urgent concern, and given that executive branch members were involved, there were “confidential and potentially privileged matters” that required further consultation.

By Wednesday, though, there was a compromise: Schiff announced that Maguire would testify in an open session of the committee on Sept. 26. Committee members will learn more about the case when Atkinson appears at a closed hearing Thursday morning.