US House of Representatives Speaker Nancy Pelosi on Sunday added a Republican critic of former president Donald Trump to a select committee inquiry on the Capitol Hill riot of January 6.

Adam Kinzinger, an Illinois Republican said he "humbly accepted" the role, saying he would "work diligently to ensure we get to the truth and hold those responsible for the attack fully accountable."

The inquiry, which is scheduled to start on Tuesday, will hear from police officers who fought the rioters and have the power to subpoena witnesses and documents.

Why is the inquiry going ahead?

Nancy Pelosi said the new investigation would try "to get the truth" about what happened on January 6 when pro-Trump supporters stormed the Capitol as its members voted to name Joe Biden as US president.

The House Speaker, who is a Democrat, said her goal was to make sure "such an attack can never happen again."

The Republican leadership has boycotted the inquiry after Pelosi excluded two Congressmen, Jim Jordan and Jim Banks, from the select committee.

"The two that I would not appoint are people who would jeopardize the integrity of the investigation and there's no way I would tolerate their antics," Pelosi said in an interview Sunday with ABC News.

As a result, House Republican leader Kevin McCarthy said the select committee was a "sham process" and withdrew his full five members from the inquiry.

Why did Kinzinger accept the role?

"Let me be clear, I'm a Republican dedicated to conservative values, but I swore an oath to uphold and defend the Constitution," Kinzinger said in a statement.

"And while this is not the position I expected to be in or sought out, when duty calls, I will always answer."

"This moment requires a serious, clear-eyed, non-partisan approach. We are duty-bound to conduct a full investigation on the worst attack on the Capitol since 1814."

He said that after "lies and conspiracy theories" of the last few months, "the American people deserve transparency and truth on how and why thousands showed up to attack our democracy."

Pelosi has said that Kinzinger would add "great patriotism" to the inquiry.

She has already appointed another Republican, Congresswoman Liz Cheney, to the select committee. Cheney, along with Kinzinger, was one of 10 Republicans who voted to impeach Trump for inspiring the Capitol Hill riot.

jc/mm (AFP, AP)