President Trump pressed the leader of Ukraine to investigate the son of former Vice President Joe Biden in a call between the two leaders that is at the center of an extraordinary whistleblower complaint, according to two people familiar with the matter.

Trump used the July 25 conversation with Ukraine President Volodymyr Zelensky to pressure the recently elected leader to more aggressively pursue an investigation that Trump believed would deliver potential political dirt against one of the president’s political adversaries, the people said.

One source familiar with the contents of the exchange said that Trump did not raise the issue of American military and intelligence aid that had been pledged to Ukraine, indicating there was not an explicit quid pro quo in that call.

The call is part of a broader set of facts included in the whistleblower complaint that is at the center of a showdown between the executive branch and Congress, with officials in the Trump administration refusing to divulge any information about the substance of an August 12 report to the inspector general of the U.S. intelligence community.

The revelation that Trump pushed Zelensky to pursue the Biden probe, which was first reported by the Wall Street Journal, represents the most detailed account so far of the president’s conduct that prompted a U.S. intelligence official to file a whistleblower action against the president.

The disclosure comes amid new details about the White House’s role in preventing Director of National Intelligence Joseph Maguire from complying with Congressional demands for the material in the complaint.

White House Counsel Pat Cipollone has been engaged in the matter since shortly after the whistleblower action surfaced, officials said, helping to identify legal obstacles to the sharing of information that could be politically damaging to Trump.

Cipollone’s involvement reveals a more direct White House role in the dispute than has previously been reported.

The issue has become a source of tension between the White House and the office of the DNI, with Maguire forced into the position of fending off lawmakers’ demands — citing jurisdictional objections — while the White House avoids asserting executive privilege or taking a clear legal position.

Maguire’s scheduled testimony in open session before the House Intelligence Committee next week could force the White House’s hand. He is almost certain to face questions about any direction he has taken from the White House.

Maguire has also turned to the Justice Department’s Office of Legal Counsel for guidance on the matter, according to a senior administration official who said, “Those consultations are ongoing. At no time has Pat Cipollone personally directed the DNI to withhold information from Congress.”

Even so, the administration has essentially rebuffed the legislative branch, arguing that the whistleblower complaint does not fall under the intelligence director’s jurisdiction, because it pertains to communications by someone outside the intelligence community.

It is now clear those communications center on Trump, who again on Friday denied that had engaged in any inappropriate discussions with world leaders but refused to say whether he brought up the Biden investigation on the call with Zelensky.

“It doesn’t matter what I discussed,” Trump said in an exchange with reporters.

Trump’s former lawyer, Rudy Giulliani, has persistently prodded Ukrainian officials in recent months to more aggressively pursue an investigation involving Biden that Ukrainian officials had abandoned and have described as baseless. Hunter Biden served as a board member on a Ukrainian energy company while his father served as vice president.