Former President Joe Biden’s White House physician refused Wednesday to answer questions for a Republican-led congressional investigation into Biden’s mental acuity.

Kevin O’Connor cited both physician-patient privilege and his Fifth Amendment right against self-incrimination, according to a statement that one of his lawyers read to the House Oversight Committee at the start of his scheduled closed-door deposition.

O’Connor, who served all four years as Biden’s doctor in the White House, had been subpoenaed by Republicans on the oversight panel who are investigating whether Biden and his aides concealed mental deficiencies that made him unable to perform presidential duties.

The inquiry includes questions about whether Biden’s staff abused an autopen, a device routinely used by presidents to put their signatures to formal documents, to illegally carry out official actions in his name.

Presidents have for decades used an autopen to sign all manner of documents, including major legislation; doing so is legal as long as a president authorizes it. President Donald Trump and his allies have been stoking a theory that Biden suffered from severe age-related decline that left him incapable of making presidential decisions at all, making any action taken in his name via the device legally invalid.

In a statement, Rep. James Comer, R-Ky., the Oversight chair, blasted O’Connor for refusing to cooperate and suggested he was trying to hide something.

“It’s clear there was a conspiracy to cover up President Biden’s cognitive decline after Dr. Kevin O’Connor, Biden’s physician and family business associate, refused to answer any questions and chose to hide behind the Fifth Amendment,” he said. “The American people demand transparency, but Dr. O’Connor would rather conceal the truth.”

Witnesses routinely invoke their Fifth Amendment rights in testimony to Congress. Allies of Trump, for example, invoked their rights when refusing to testify to the committee that investigated the Jan. 6, 2021, attack on the Capitol by a mob of his supporters.

In a statement to the committee, O’Connor’s lawyer David Schertler cited Trump’s own invocation of the Fifth Amendment in other contexts and noted that the Supreme Court has said that one of the most basic functions of the amendment is to protect “innocent people who might otherwise be ensnared in ambiguous circumstances.”

He noted that Trump has directed the Justice Department to conduct a parallel investigation into Biden’s use of an autopen and that Comer had suggested something was criminal about the matter.

“The pending Department of Justice criminal investigation leaves Dr. O’Connor no choice but to invoke his constitutional rights under the Fifth Amendment of the Constitution to any questions posed by the committee,” Schertler said in his statement. “We want to emphasize that asserting the Fifth Amendment privilege does not imply that Dr. O’Connor has committed any crime.”

He also invoked physician-patient privilege, which sometimes lets doctors avoid disclosing confidential medical information for legal proceedings. O’Connor’s lawyers had sent a letter to the committee last weekend raising concerns about whether Republicans would respect that privilege.

The committee has subpoenaed many former top Biden aides, including Ron Klain, Mike Donilon, Anita Dunn, Bruce Reed and Steve Ricchetti. The first to testify before the committee last month was Neera Tanden, who had served as Biden’s staff secretary and then his domestic policy adviser.

O’Connor is the second aide to Biden who has declined to answer questions in the investigation. Anthony Bernal, a senior adviser to former first lady Jill Biden, did not appear for a transcribed interview that was scheduled last month.

Rep. Robert Garcia of Long Beach, the top Democrat on the Oversight panel, dismissed the Republican investigation as a waste of time.

“Oversight Republicans could be working to lower costs for American families and conducting oversight of President Trump’s corruption, but instead are obsessed with the past,” he said.

Comer has said his committee will release a report of all its findings after the probe is complete.

The Associated Press contributed to this report.