By Ted Barrett, CNN

(CNN) — Another senior aide to former President Joe Biden is testifying Thursday before the House Oversight Committee as part of the GOP-led panel’s investigation into what it charges was a “cover-up” of Biden’s mental decline.

The committee has said it is also investigating the “potentially unauthorized issuance of sweeping pardons and other executive actions.”

Ian Sams, who served as a special assistant to the president and senior adviser in the White House Counsel’s Office and later worked for Kamala Harris’ presidential campaign, is appearing voluntarily.

Sams arrived at the deposition and declined to answer reporters’ questions.

House Oversight Chair James Comer was asked if he expects Sams to cooperate during the session.

“He had a lot to say while he was working for the White House. So, I’m sure he’ll have a lot to say today. He was very vocal and very active on Twitter. So we’re going to ask him, you know, a lot of questions about what he would say publicly from behind the podium at the White House, and what he would tweet about and see if he still stands by a lot of those those claims he made,” he said.

Several witnesses, including Biden’s White House physician, Dr. Kevin O’Connor, were subpoenaed and then invoked their Fifth Amendment right not to answer questions.

Other aides have fiercely defended Biden.

Anita Dunn, the former senior adviser to the president for communications, said in her opening statement obtained by CNN, “While I observed that President Biden aged physically during his time in o?ce, which is something that happens to every president, he remained throughout my interactions with him fully engaged and clear in his directions and supervision.”

She added: “I did not observe White House sta? making key decisions or exercising the powers of the presidency without President Biden’s knowledge or consent.”

Steve Ricchetti, a former Biden counselor, said he was cooperating with the inquiry “because I believe it is important to forcefully rebut this false narrative about the Biden presidency and our role in it,” in a statement obtained by CNN and delivered to the committee as part of his closed-door testimony.

“There was no nefarious conspiracy of any kind among the president’s senior staff, and there was certainly no conspiracy to hide the president’s mental condition from the American people,” Ricchetti said.

CNN reported last month that Ron Klain, who had served as chief of staff to Biden, told the committee that former National Security Adviser Jake Sullivan and former Secretary of State Hillary Clinton had raised concerns to him about Biden’s political chances as he sought reelection.

Sams did not respond to CNN for a request for comment about his testimony.

CNN’s Annie Grayer contributed to this report.

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