WASHINGTON — 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.

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.

— The Associated Press