A senior US 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 US Capitol.

Reeker told lawmakers that in the June-August period he had no knowledge of the possibility that US 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 US 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.

US 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 US 2016 presidential election.

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

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 US 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.”