Joe Biden’s White House doctor’s uncooperativeness in a House probe looking into the former president’s fitness to serve has led the Republican congressman overseeing the investigation to allege a “coverup” of Biden’s “cognitive decline.”

“It’s now 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,” House Oversight Committee Chairman James Comer, R-Ky., said in a statement.

“The American people demand transparency but Dr. O’Connor would rather conceal the truth. Dr. O’Connor took the Fifth when asked if he was told to lie about President Biden’s health and whether he was fit to be President of the United States,” Comer continued.

Republicans on the Oversight Committee subpoenaed O’Connor last month as part of a their sweeping investigation into Biden’s health and his mental fitness as president. They claim some policies carried out during Biden’s term through the use of the White House autopen may be illegitimate if it’s proven the Democrat was mentally incapacitated for some of his term, the Associated Press reported.

Biden has strongly denied that he was not in a right state of mind at any point while in office, calling the claims “ridiculous and false.”

Comer suggested “legislative solutions to prevent such a coverup from happening again,” but did not say what those measures would be.

O’Connor’s testimony is not the end of the probe, Comer said.

“We will continue to interview more Biden White House aides to get the answers Americans deserve.”

The Associated Press contributed to this report.