House Speaker Nancy Pelosi announced plans for Congress to establish an outside and independent commission to investigate "the facts and causes" related to the attack on the U.S. Capitol on Jan. 6.

In a letter sent to her Democratic colleagues on Monday, the California Democrat said the commission will be modeled on the commission established after the Sept. 11, 2001, terrorist attacks.

Pelosi noted the recent work of retired Lt. Gen. Russel Honoré, who has "been assessing our security needs by reviewing what happened on January 6 and how we must ensure that it does not happen again."

"As we prepare for the Commission, it is also clear from General Honoré's interim reporting that we must put forth a supplemental appropriation to provide for the safety of Members and the security of the Capitol," Pelosi wrote.

Pelosi had first suggested the creation of such a commission two weeks ago.

Such a move will require legislation and will likely tee up partisan difficulties.

Her letter to colleagues came several hours after four House Republicans sent a letter to Pelosi suggesting she may be responsible for the delay in the deployment of National Guard troops ahead of and during the insurrection. The letter did not mention Senate Minority Leader Mitch McConnell, who on Jan. 6 was still the Senate majority leader and would have also had a say in the Capitol's security posture.

"Five weeks have passed since the January 6th attack on the Capitol building, and many important questions about your responsibility for the security of the Capitol remain unanswered," their letter reads.

Drew Hammill, Pelosi's deputy chief of staff, called the Republicans' letter a "transparently partisan attempt to lay blame on the Speaker."

"The Speaker has and will continue to take action to ensure accountability and enhance the security of the Capitol," he said in a statement. "Following the insurrection, the House Sergeant at Arms, the Senate Sergeant at Arms and the Chief of the Capitol Police were removed from their positions. It is the job of the Capitol Police Board, on which these three individuals sat, to properly plan and prepare for security threats facing the U.S. Capitol."

Sen. Chris Coons, a close ally of President Biden, told ABC's This Week he supports a Sept. 11-style commission to probe further into the events leading up to the attack.

"There's still more evidence that the American people need and deserve to hear," the Delaware Democrat said. "The 9/11 Commission is a way to make sure that we secure the Capitol going forward, and that we lay bare the record of just how responsible and how abjectly and violating of his constitutional oath President Trump really was."

Following the attack on the Capitol, heightened security measures were deployed around the complex, including the requirement of members to walk through metal detectors and various forms of fencing secured around the Capitol's perimeter.

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