WASHINGTON (AP) — Former national security adviser John Bolton failed to appear for an interview with impeachment investigators Thursday, making it unlikely that he will provide testimony to the House about President Donald Trump's handling of Ukraine.

Democrats indicated they have no interest in a drawn-out court fight over Bolton's testimony or that of any others as they move into a more public phase of their impeachment inquiry. They say they will simply use the no-shows as evidence of the president's obstruction of Congress.

An attorney for Bolton, Charles Cooper, said his client had not received a subpoena. Cooper had said Bolton wouldn't appear without one.

An aide to Vice President Mike Pence did appear under subpoena Thursday to speak with impeachment investigators and was deposed for more than four hours.

Jennifer Williams, a career foreign service officer detailed to Pence's office from the State Department, is one of several White House aides who were listening in on a July phone call between Trump and Ukraine President Volodymyr Zelenskiy in which Trump asked the new leader to investigate Democrats, according to an administration official who requested anonymity to discuss the conversation.

That call, in which Trump asked Zelenskiy to investigate political rival Joe Biden and his family and also Ukraine's role in the 2016 U.S. presidential election, is at the center of the Democrats' impeachment probe.

Though Trump has said there was no "quid pro quo," several of the witnesses, including top Ukraine diplomat William Taylor, have testified that it was their understanding that Ukraine would not receive military assistance or a coveted Oval Office visit until it met the president's demands.

Democrats say the refusal of witnesses like Bolton, Mulvaney and Perry to appear — under Trump's orders — will add fuel to their case that the president has obstructed justice. They say obstruction is likely to be an article of impeachment against Trump, when and if they are written.

Lawmakers leaving the deposition said that Williams' testimony had lined up with the accounts of others.

"It just never ceases to amaze me how all of these people in every material aspect corroborate one another," said Washington Rep. Denny Heck, a member of the House intelligence panel, as he left the interview.

Republican Rep. Mark Meadows of North Carolina said that lawmakers were finding out more details about Pence's September visit to Warsaw, Poland, where he met Zelinskiy. Pence has said that he and the Ukrainian president did not discuss Biden during their closed-door meeting, but they had discussed the White House's decision to halt security aid to the nation meant to counter Russian aggression.

Speaking to reporters in New Hampshire, Pence stood by Trump and said if Americans read the administration's rough transcript of the call they will find "there was no quid pro quo, the president did nothing wrong." Pence called the impeachment inquiry a "disgrace."

Investigators are wrapping up the private interviews as they prepare to start public hearings next week. House Intelligence Committee Chairman Adam Schiff announced Wednesday that three State Department witnesses will appear in two hearings next Wednesday and Friday: Taylor, career department official George Kent and Marie Yovanovitch, the former U.S. ambassador to Ukraine. Yovanovitch was ousted in May on Trump's orders and Taylor replaced her; both have testified about their concerns with the administration's policy on Ukraine.

Democrats scheduled 13 witnesses to testify behind closed doors this week as they have wrapped up the private phase of the probe, but so far only Williams and another State Department employee, David Hale, have shown up. Trump has directed his employees not to cooperate with the investigation.

In addition to Bolton, Democrats had requested interviews from two other high-level Trump administration witnesses, acting White House Chief of Staff Mick Mulvaney and Energy Secretary Rick Perry. Perry did not show up for his Wednesday interview and Mulvaney was not expected to appear for his scheduled deposition on Friday.

Still, Democrats have indicated they think they already have ample testimony about Trump's conduct on Ukraine. The slew of current and former officials from the State Department and White House have appeared and largely corroborated the same narrative — that Trump had delegated his lawyer, Rudy Giuliani, to guide U.S.-Ukraine policy and that the two men were focused on pressuring Ukraine as the administration withheld military aid from the country.

Heck said there is "already a mountain of evidence" in the probe.

"I think there's more evidence to the effect that the president shook down Ukraine, tried to cover it up, and threatened to and then withheld security assistance to Ukraine than there is evidence that the sun will come up in the East tomorrow," Heck said.

Williams is the first person directly connected with Pence to testify in the probe.

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Associated Press writers Jill Colvin, Eric Tucker, Matthew Lee and Lisa Mascaro contributed to this report.