House Oversight Committee Chair James Comer said Thursday on "Jesse Watters Primetime" that four former Biden staffers as well as former White House physician Kevin O'Connor have been called in for questioning, adding the committee wants to depose the "unelected bureaucrats" from the former administration.

Former President Joe Biden's mental decline during his term has been under the media spotlight, with CNN's Jake Tapper and Axios' Alex Thompson releasing a new book detailing his struggles. With Republicans demanding accountability from Democrats who they say covered for Biden, Fox host Jesse Watters asked Comer whom he plans to subpoena first.

"Well, we're going to request that four staffers that we've identified as the main gatekeepers for the Biden White House, that they come and answer our questions. We're also requesting that Dr. [Kevin] O'Connor come. That's the first step towards a subpoena," Comer said.

In addition to concerns over Biden's mental health, his office released a statement Sunday saying he had been diagnosed with "a more aggressive form" of prostate cancer, which has already metastasized to the bone.

Watters asked the lawmaker to name the four former staffers he wants questioned.

"Well, they're no names that anyone that normally watches Fox would know," Comer said. "They're behind the scenes. They fly under the radar. These are the unelected bureaucrats that we believe had an overwhelming influence over Joe Biden and could have possibly been serving as de facto presidents of the United States."

"The first step to issue a successful subpoena is to offer someone an invitation, a sincere request to come in and answer questions. If they do not, the next step will be the subpoenas. I agree with everything that the last person said about subpoenas, except the special counsel," Comer said. "I think that we can do that in Congress. We're going to bring these people in for depositions and transcribed interviews, and we're going to ask every question that a good journalist like yourself, Jesse, would ask. The American people want to know this. Momentum's on our side. New evidence has emerged just in the last few days, and I think that all eyes will be watching this new investigation by the House Oversight Committee."

Republicans voiced concerns about Biden's mental decline before he even entered the 2020 race. Those concerns increased during Biden's time in the White House, as he was seen in videos stumbling and struggling to speak. Democrats and media pundits dismissed the criticism. Former White House Press Secretary Karine Jean-Pierre notably labeled the clips "cheap fakes," claiming they were edited to make Biden look bad.

Details from Tapper's and Thompson's book have again placed Biden's presidency under scrutiny, revealing that at one point his health declined so severely that aides discussed putting him in a wheelchair.