By MATT BROWN and MICHELLE L. PRICE, Associated Press

WASHINGTON (AP) — Republicans on the House Oversight Committee issued a subpoena Thursday to Anthony Bernal, a senior aide to former first lady Jill Biden, as part of their rapidly expanding investigation into former President Joe Biden’s mental fitness while in office.

The subpoena signed by Rep. James Comer of Kentucky, the Republican Oversight chairman, requires Bernal to appear for a deposition on July 16. It came after several weeks of back-and-forth with Bernal’s lawyer over the timing of a voluntary interview.

“Given your close connection with both former President Biden and former First Lady Jill Biden, the Committee sought to understand if you contributed to an effort to hide former President Biden’s fitness to serve from the American people,” the subpoena reads. “To avoid any further delays, your appearance before the Committee is now compelled.”

Bernal is the second former Biden staffer to be subpoenaed by the committee and unlikely to be the last. The committee this week heard voluntary testimony from Neera Tanden, a former director of Biden’s domestic policy counsel, and is intent on securing interviews with several other members of Biden’s inner circle as part of its investigation.

Bernal did not respond Thursday to a message seeking comment.

Comer has also subpoenaed Kevin O’Connor, who served as Biden’s physician at the White House. O’Connor will testify before the committee on July 9.

It’s all part of a remarkable Republican effort, supported by President Donald Trump, to investigate the last occupant of the Oval Office six months after he left office. Trump himself has ordered White House lawyers and the Justice Department to investigate Biden, questioning the legitimacy of his use of the autopen to sign pardons and other documents.

Boosting the GOP investigation, Trump has waived executive privilege for eight former Biden administration officials to testify to Congress, including Bernal, according to a White House official who was not authorized to speak publicly and insisted on anonymity.

With the privilege lifted, former staffers are free to discuss their interactions with Biden while he was president.

In addition to Bernal, executive privilege has been waived for Biden White House senior advisers Mike Donilon and Anita Dunn, former White House chief of staff Ron Klain, former deputy chief of staff Bruce Reed, former counselor to the president Steve Ricchetti, former deputy chief of staff Annie Tomasini and a former assistant to the president, Ashley Williams. Comer is seeking interviews with all of them.

Democrats have dismissed the inquiry into Biden’s mental state as a partisan exercise that distracts from other pressing issues.

Rep. Wesley Bell, a Missouri Democrat who sits on the Oversight committee, said after the interview with Tanden that it “was an extraordinary waste of time” and produced “no new evidence.”

Bell said lawmakers should focus on issues like the costs of food, housing and potential changes to healthcare policy rather than “dig up some kind of post-impeachment, or whatever we’re doing here.”

The unfolding investigation has significant implications for politics and policy. Republican lawmakers have argued that any executive actions or policies enacted through the autopen procedure could be found invalid if Biden were somehow incapacitated or not of a sound state of mind while in office.

Trump and his allies have claimed, without evidence, that Biden was not aware of the actions his administration had taken on a range of issues, including presidential pardons, environmental policy and labor rights, among other issues.

A move to reverse such executive actions, which would almost certainly face legal scrutiny and a battle in the courts, could impact scores of executive orders taken throughout Biden’s term.

“Let this subpoena send a clear message to Biden’s inner circle: We will stop at nothing to expose the truth about Joe Biden’s decline (and) unauthorized use of the autopen,” Comer wrote on social media.

Trump this month directed his administration to investigate Biden’s actions as president, arguing that aides engaged in a “conspiracy” to mask Biden’s mental state. Senate Republicans have also launched their own inquiries.

“We need to know who was in charge during the last months of the Biden administration. Was it his wife, his chief of staff, nameless others? None of these people were elected by the American people, nor were they authorized by the Constitution and laws of the United States to carry out the duties of the president of the United States,” Sen. John Cornyn, R-Texas, said during a June hearing.

Associated Press writer Darlene Superville contributed to this report.