(Bloomberg) -- The House select committee investigating the Jan. 6 assault on the U.S. Capitol by a pro-Trump mob on Monday asked three additional Republican congressmen to testify about what they knew regarding events leading up to the riot.

Representatives Mo Brooks of Alabama, Andy Biggs of Arizona and Ronny Jackson of Texas were sent letters Monday from Committee Chairman Bennie Thompson seeking their cooperation. 

They have not been subpoenaed. So far, the committee has declined to issue subpoenas to their fellow members of Congress, which likely would result in a protracted legal confrontations.

“We urge our colleagues to join the hundreds of individuals who have shared information with the Select Committee to get to the bottom of what happened on January 6th,” Thompson and Liz Cheney, a Wyoming Republican and vice chair of the committee, said in a statement.

Brooks, who is running this fall for a Senate seat, said in a March television that former President Donald Trump had asked him to “rescind the 2020 elections,” remove Joe Biden from the White House, and help reinstall him to the White House.

That remark came after Trump had revoked his endorsement of Brooks’ Senate candidacy. Brooks has denied he had a role in organizing the rally on Jan. 6 that immediately preceded the riot and has said in the past he might testify if asked.

Biggs, a former chairman of the conservative House Freedom Caucus, is among several lawmakers who had had discussions or communications with Trump’s last chief of staff, Mark Meadows, about ways to overturn the election, according to testimony also given to the committee. 

Jackson of Texas -- who was been a Trump White House doctor before being elected to Congress -- -- has come under the spotlight with news that members of the far-right Oath Keepers exchanged messages about his safety during the attack on the Capitol. The messages, revealed in a court filing in April, included one that alleged Jackson needed protection by the group because he had “critical data to protect.”

Other House Republican lawmakers have previously declined invitations to appear before the committee, including Republican leader Kevin McCarthy. 

Thompson said last week that the committee would be issuing new invitations this week to McCarthy, Representatives Jim Jordan of Ohio and Scott Perry of Pennsylvania, and possibly others.

Thompson says the panel expects to wind up its mostly closed-door witness interviews as it heads into eight planned hearings in June, and a final report of its findings in early fall. The committee says it is has spoken to about 800 people.

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