Former national security adviser John Bolton did not show up Thursday morning for a scheduled Trump impeachment inquiry deposition with lawmakers, though a State Department aide detailed to Vice President Pence's office appeared for testimony after Democrats issued a subpoena.

A House Intelligence Committee official told Fox News suggested they will not issue a subpoena to compel Bolton's testimony, as the former adviser's attorney said they would challenge one in court, which could be a lengthy process.

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"We would welcome John Bolton’s deposition and he did not appear as he was requested today," the official said. "His counsel has informed us that unlike three other dedicated public servants who worked for him on the NSC and have complied with lawful subpoenas, Mr. Bolton would take us to court if we subpoenaed him."

The official added: "We regret Mr. Bolton’s decision not to appear voluntarily, but we have no interest in allowing the administration to play rope-a-dope with us in the courts for months. Rather, the White House instruction that he not appear will add to the evidence of the president’s obstruction of Congress."

Bolton had become a key witness for the House Intelligence, Oversight, and Foreign Affairs Committees, especially after the testimony of acting U.S. Ambassador to Ukraine Bill Taylor, who testified that Bolton was opposed to Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, which led to the anonymous whistleblower complaint that sparked the inquiry.

“Ambassador Bolton was not interested in having — did not want to have the call because he thought it was going to be a disaster,” Taylor testified, according to a newly-released transcript. “He thought that there could be some talk of investigations, or worse, on the call."

Bolton, according to Taylor’s testimony, was concerned during his time at the White House about Trump’s actions toward Ukraine, and abruptly ended a meeting with U.S. Ambassador to the EU Gordon Sondland, supposedly referring to a possible connection between an Oval Office meeting for Zelensky and investigations into the 2016 election and the Bidens as a "drug deal."

Bolton is not the first witness to get away with rejecting the House committees' request. His former deputy, Charles Kupperman, who is also represented by Cooper, failed to comply with a House subpoena, seeking a court order to determine whether he should testify in accordance with Democrats' demands or follow instructions from the White House to remain silent based on an assertion of privilege. House Democrats withdrew their subpoena before the court made a decision. Now it appears they are similarly uninterested in engaging in a court battle.

As for Jennifer Williams, Pence's special adviser for Europe and Russia, Fox News has been told that the White House tried to prevent or limit her testimony. But now that she has been served with a subpoena, she is complying.

"Jennifer is a longtime dedicated State Department employee. If required to appear, she will answer the Committees' questions," her attorney Justin Shur had said prior to her testimony. "We expect her testimony will largely reflect what is already in the public record."

According to a Trump administration official, Williams was on the July 25 phone call between Trump and Zelensky.

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Several officials failed to appear for scheduled depositions earlier this week: John Eisenberg, the lead attorney for the National Security Council; Robert Blair, a top aide to acting White House chief of staff Mick Mulvaney; National Security Council aide Michael Ellis, and Office of Management and Budget aide Brian McCormack all skipped their scheduled appearances Monday.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said the no-shows could be used as “evidence to support a possible obstruction of Congress charge against the president.

Fox News’ Mike Emanuel, John Roberts, Chad Pergram, Gillian Turner and Brooke Singman contributed to this report.