Cooper’s testimony could fill in details of an explosive aspect of Democrats’ impeachment inquiry: whether Trump withheld nearly $400 million in military aid for Ukraine to pressure the ally’s new president, Volodymyr Zelensky, to open two politically motivated investigations into Trump’s Democratic adversaries. The hold on aid came amid Ukraine’s struggle to fend off Russian aggression in Crimea and despite authorization from Congress and the Defense Department to allocate the funds.

Trump’s budget office, instead, blocked the aid in late July — news first revealed by POLITICO on Aug. 29. The administration lifted the hold on the aid on Sept. 11, as bipartisan scrutiny of the decision mounted.

Democrats were largely tight-lipped emerging from Cooper’s deposition. However, Rep. Mark Meadows (R-N.C.), a conservative ally of the president, said elements of her testimony conflicted with that of William Taylor, the top U.S. diplomat in Ukraine who testified a day earlier. His testimony provided the first direct link between Trump and an effort to tie the military aid to politically motivated investigations. Meadows declined to detail the purported discrepancies.

Cooper’s testimony comes despite the Pentagon blowing off a deadline last week to comply with a subpoena for documents related to the episode.

Cooper was also the first Defense Department witness to defy a directive not to testify, a sign that Trump’s blockade of Democrats’ impeachment inquiry has continued to erode. Several senior State Department officials and a former National Security Council official have already taken the same route.

On Wednesday, three Defense officials indicated they were collecting and reviewing documents in response to the House subpoena but were sending them to the White House, rather than Congress, to screen for potential privileged materials.

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The White House, however, has vowed to resist any attempt by Democrats to seek materials for their ongoing impeachment probe.

The process of collecting documents began Oct. 3, when the Pentagon’s top lawyer first learned of the inquiry and ordered the department to preserve records related to it, the three defense officials said.

“There are thousands and thousands of pages that are potentially responsive,” one of the Defense officials said. But “we don’t own a lot of the documents that are particularly of interest here.”

Aside from Cooper, no other Pentagon officials have been subpoenaed, the officials added.