WASHINGTON — House Democrats are asking 10 federal agencies for documents and communications from the government as part of a wide-ranging investigation into the attack on the Capitol and how it happened.

Seven House committees that are looking into the Jan. 6 insurrection sent letters Thursday to the agencies and to Washington, D.C., Mayor Muriel Bowser, Washington’s Metropolitan Police Department, the Capitol Police and other security officials.

The Democratic-led committees are ramping up their investigations as hopes for a bipartisan, independent commission to study the attack are fading. House Speaker Nancy Pelosi had proposed legislation that would create the panel, which would be modeled after a similar commission following the Sept. 11 attacks, but Republicans rejected the proposal, complaining that it would be overly tilted toward Democrats.

Pelosi said Thursday that she would still like to have a bipartisan commission but that there were other ways to investigate the riot. She also said she had called on House committees to look into the attack, which was led by supporters of former President Donald Trump who sought to stop lawmakers from certifying Joe Biden’s election.

“We have to find the truth,” Pelosi said. “And we will, and we’re not walking away from that.”

The committees sent letters to the White House, Office of the Director of National Intelligence, National Archives, FBI, National Guard Bureau, the U.S. Park Police and the departments of Justice, Defense, Interior and Homeland Security. They asked for documents and communications between early December and Biden’s Jan. 20 inauguration about preparations for protests, discussions about the electoral count and any action related to the events of Jan. 6 and its aftermath.

The House panels join Senate committees that are also looking into the insurrection. The Senate Homeland and Governmental Affairs Committee and the Senate Rules Committee have already held two hearings with security officials about what went wrong as the rioters broke into the Capitol and sent lawmakers fleeing for their lives. The security officials, including from the Capitol Police and the Department of Defense, have pointed fingers at each other as they described violent attacks on overwhelmed police officers and desperate pleas for backup.

The Democratic and Republican leaders of those Senate committees said in a statement Thursday that they will continue to investigate the attacks and are conducting interviews of many of the officials involved.

“The Committees expect to release a bipartisan report on our investigation in the coming months,” said Senate Rules Committee Chair Amy Klobuchar, D-Minn., and Senate Homeland Chair Gary Peters, D-Mich., along with Republican Sens. Roy Blunt of Missouri and Rob Portman of Ohio. “We are committed to providing the American people, including those who work in the Capitol, with the answers they deserve as well as recommendations to ensure this never happens again.”

House Republicans objected to Pelosi’s plan for a commission because it would be comprised of more Democrats than Republicans, unlike the 9/11 panel. She has said she would be willing to negotiate on that, but not on the scope of the investigation, which Republicans had also objected to.

The legislation does not mention Trump or his calls for his supporters to “fight like hell” to overturn his election defeat, but Republicans criticized the broad latitude that the commission would have to investigate the causes of the insurrection. They also objected to a series of findings in the bill that quoted FBI Director Christopher Wray saying that racially motivated violent extremism, and especially white supremacy, is one of the biggest threats to domestic security.

More than 300 people have been charged in connection to the riot. Authorities have said they believe at least 100 more could face charges.

Pelosi said she wants the committee investigations to be bipartisan, but it is unclear the extent to which Republicans will participate. The House panels looking into the attack are the Intelligence; Judiciary; Oversight and Reform; Armed Services; Appropriations; Homeland Security; and Administration committees.

The ramped-up investigation comes as Capitol officials are improving the building’s physical security, including reinforcing the House doors that the rioters attempted to breach. During the insurrection, a woman was shot to death by police as she tried to climb through a broken window adjacent to the chamber, and rioters banged on the main door to the House as lawmakers were trapped inside. She was among five people who died from the riots.

Expediting the “hardening of vulnerable windows and doors” was among the many recommendations from a review led by retired Army Lt. Gen. Russel Honoré after the siege. The changes had been planned before Jan. 6 and are now underway, according to a person familiar with the security plans who discussed them on condition of anonymity.

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AP Congressional Correspondent Lisa Mascaro contributed to this report.