(Reuters) - A senior U.S. diplomat told lawmakers on Saturday he did not know whether President Donald Trump had withheld aid for Ukraine to force an investigation of a political rival, two sources said, even as Democrats said he corroborated evidence gathered in their impeachment probe.

Philip Reeker, the acting assistant secretary of state for European and Eurasian affairs, spent about eight hours with the House of Representatives Foreign Affairs, Intelligence and Oversight committees behind closed doors at the U.S. Capitol.

Reeker told lawmakers that in the June-August period he had no knowledge of the possibility that U.S. security aid to Ukraine may have withheld to pressure Kiev to launch investigations that could have helped Trump’s 2020 re-election bid, as some have alleged, according to the sources familiar with the matter.

Reeker knew the aid had been withheld, but not why, said the sources, who requested anonymity. Reeker testified he heard resistance to releasing the aid was coming from acting White House Chief of Staff Mick Mulvaney, one of the sources said.

At the end of Saturday’s session, Intelligence Committee Chairman Adam Schiff told reporters lawmakers were making “rapid progress” in the impeachment inquiry. He declined to say when the panels might advance to the next phase of hearings open to the public.

The Democratic-led committees are conducting an inquiry focusing on Trump’s request to Ukrainian President Volodymyr Zelenskiy that he investigate former U.S. Vice President Joe Biden, a leading Democratic presidential candidate, and his son Hunter Biden, who had served on the board of a Ukrainian gas company.

U.S. election law prohibits candidates from accepting foreign help in an election.

As part of their probe, the lawmakers are examining whether Trump withheld $391 million in security assistance until Zelenskiy publicly committed to both an investigation of the Bidens and a debunked conspiracy theory that Ukraine - not Russia - meddled in the U.S. 2016 presidential election.

Democrats emerging from the session said Reeker’s deposition corroborated previous testimony taken by the panels.

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But they offered no details and it was not clear what Reeker may have corroborated.

Representative Stephen Lynch told reporters that Reeker, whose portfolio includes Ukraine and Russia, was a “much richer reservoir of information than we originally expected.”

But Reeker, according to the sources, was largely out of the loop on policy toward Ukraine, leaving it to Kurt Volker, then the U.S. special representative for Ukraine negotiations, and others.

Republican Representative Mark Meadows, speaking to reporters as a long day of testimony was winding down, said it “was a good day for the president” and described Reeker as “a top level official not giving any potential incriminating evidence.”

FOCUS ON UKRAINE

Reeker submitted to questioning in response to a congressional subpoena issued after the State Department attempted to block his testimony, according to an official working on the impeachment inquiry.

The testimony was the first since a federal judge on Friday rejected a claim by Trump and his Republican allies that the Democrats’ impeachment inquiry process was illegitimate because the full House had not voted to authorize it.

House Speaker Nancy Pelosi launched the formal Trump probe on Sept. 24 under her broad powers as speaker.

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At the heart of the inquiry is a July 25 phone call in which Trump asked Zelenskiy to investigate the Bidens and a claim - debunked as a conspiracy theory - that a Democratic National Committee computer server was in Ukraine.

William Taylor, the top U.S. diplomat in Ukraine, testified on Tuesday that Trump made the withheld aid contingent on Zelenskiy announcing the requested investigations.

Trump denies wrongdoing and, backed by his fellow Republicans in Congress, insists he is being treated unfairly.

Reeker, 54, a career diplomat, is the latest in a growing list of current and former officials who have met with investigators.

Among the issues the committees explored with Reeker was Trump’s abrupt dismissal in May of the ambassador to Ukraine, Marie Yovanovitch. According to emails given to congressional committees this month, Reeker sought to intervene when Trump supporters accused Yovanovitch of being disloyal to the president.

Reeker sought a stronger State Department defense of Yovanovitch that was never provided, according to the two sources.

The committees have scheduled several depositions for next week, all behind closed doors.

Charles Kupperman, a former deputy national security adviser to Trump, who was called to appear on Monday, has asked a court whether he should comply with a congressional subpoena or honor the administration’s instruction not to testify.

The Democrats leading the inquiry said on Saturday their subpoena was valid and they expected him to appear. In a responding letter, a lawyer for Kupperman said the “proper course” would be to let the court resolve the dispute between Congress and the administration.

On Tuesday, lawmakers hope to hear from Alexander Vindman, the White House National Security Council’s top Ukraine expert.

Kathryn Wheelbarger, acting assistant secretary of defense for international security, is scheduled to appear on Wednesday, and Tim Morrison, a top White House adviser on Russia and Europe, is scheduled for Thursday.