The White House is opening its probe of former President Joe Biden's use of an autopen for commutations and other matters over a period of years.

The investigation is to be conducted by the White House counsel's office.

A senior administration official told Fox News that the probe is anticipating a review of up to one million documents.

It comes as Trump's Republican allies in the House are gearing up their own probe of the president's use of an autopen on a series of pardons and commutations, including some high-profile ones at the end of his presidency.

House Oversight Chair James Comer (R-Ky.) has subpoenaed former Biden administration aides in his probe, and the Justice Department has an open investigation.

Trump has regularly pointed to the issue since the first reports of Biden's use of the device, claiming his predecessor was incompetent and that many of his actions were invalid.

The move comes after Biden spoke to the New York Times and said he 'decided' on the commutations that bore his signature.

'I made every decision,' he told the paper, which also reviewed internal email traffic among senior aides who met with the president about commutations, then sent around emails to keep a record of what happened.

Biden knocked down the sentences of about 4,000 federal convicts, while issuing 'preemptive' pardons of Trump targets including former chairman of the joint chiefs Gen. Mark Milley and members of the House January 6th committee.

'Everybody knows how vindictive he is, so we knew that they'd do what they're doing now,' Biden said. 'I consciously made all those decisions.'

Trump, however, has repeatedly claimed that Trump was experiencing mental decline and had 'no idea' what he was doing.

'The autopen I think is maybe one of the biggest scandals that we've had in 50 to 100 years. This is a tremendous scandal,' Trump told reporters in the Oval Office Monday.

'I guarantee you he knew nothing about what he was signing. I guarantee it.'

Some of the pardons, including those in the final hours of Biden's term, followed extensive public reporting about whether he would do it.

The move comes days after Biden's former White House physician, Dr. Kevin O'Connor, repeatedly pleaded his Fifth Amendment rights against self-incrimination during an appearance before Comer's committee.

Comer blasted the move in a blistering statement. 'Most people invoke the Fifth when they have criminal liability. And you know, that's what it would appear (to be) on the surface here,' he said.

'Joe Biden was the worst, most incompetent, and senile president in our country's history,' press secretary Karoline Leavitt told the network Tuesday.

'It has been widely reported that Joe Biden handed the power of the presidency to an autopen controlled by unelected leftist staffers, who were allowed to make terrible decisions that destroyed our country.'

She said the White House was 'committed to finding the answers to the many outstanding questions the American people still have about how business in the Biden White House was conducted.

Biden, 82, who is battling prostate cancer and working on a memoir, told the Times he used the device because there were too many commutations for him to have done individually.

'We're talking about a whole lot of people,' he said.'