U.S. House Speaker Nancy Pelosi said on Monday lawmakers will establish a commission to review the "facts and causes" of the "domestic terrorist attack" on the U.S. Capitol on January 6 and issues related to the interference with the peaceful transfer of power.

She said Congress will establish an independent, September 11-style commission to look into the deadly insurrection. Meanwhile, the House will put forth supplemental spending to boost security at the Capitol.

After former President Donald Trump's acquittal at his second Senate impeachment trial, bipartisan support appeared to be growing for an independent commission to examine the deadly insurrection.

Investigations into the riot were already planned, with Senate hearings scheduled later this month in the Senate Rules Committee. Pelosi asked retired Army Lt. Gen. Russel Honore to lead an immediate review of the Capitol's security process.

In her letter to the colleagues on Monday, Pelosi said, "It is clear from his findings and from the impeachment trial that we must get to the truth of how this happened."

She added, "As we prepare for the commission, it is also clear from General Honore's interim reporting that we must put forth a supplemental appropriation to provide for the safety of members and the security of the Capitol."

More than a month after the attack that left five dead as pro-Trump supporters stormed the U.S. Capitol and sought to disrupt the certification of Joe Biden's victory as president, the Capitol complex remains guarded by more than 5,000 National Guard troops and ringed with eight-foot fences rimmed with razor wire. The troops are expected to remain through mid-March.

Lawmakers from both parties, speaking on Sunday's news shows, signaled that even more inquiries were likely. The Senate verdict on Saturday, with its 57-43 majority falling 10 votes short of the two-thirds needed to convict Trump, hardly put to rest the debate about the Republican former president's culpability for the January 6 assault.

"There should be a complete investigation about what happened," said Louisiana Sen. Bill Cassidy, one of seven Republicans who voted to convict Trump. "What was known, who knew it and when they knew, all that, because that builds the basis so this never happens again."

An independent commission along the lines of the one that investigated the September 11 attacks would probably require legislation to create. That would elevate the investigation a step higher, offering a definitive government-backed accounting of events. Still, such a panel would pose risks of sharpening partisan divisions or overshadowing President Joe Biden's legislative agenda.

House prosecutors who argued for Trump's conviction of inciting the riot said Sunday they had proved their case. They also railed against the Senate's Republican leader, Mitch McConnell, and others who they said were "trying to have it both ways" in finding the former president not guilty but criticizing him at the same time.

A close Trump ally, Sen. Lindsey Graham voted for acquittal but acknowledged that Trump had some culpability for the siege. Graham said he looked forward to campaigning with Trump in the 2022 election, when Republicans hope to regain the congressional majority.

"His behavior after the election was over the top," Graham said. "We need a 9/11 commission to find out what happened and make sure it never happens again."

The conviction tally was the most bipartisan in American history but left Trump to declare victory and signal a political revival while a bitterly divided GOP bickered over its direction and his place in the party.

The Republicans who joined Cassidy in voting to convict were Sens. Richard Burr of North Carolina, Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah, Ben Sasse of Nebraska and Pat Toomey of Pennsylvania.

"It's frustrating, but the founders knew what they were doing and so we live with the system that we have," Democratic Del. Stacey Plaskett, a House prosecutor who represents the Virgin Islands, said of the verdict, describing it as "heartbreaking." She added, "But, listen, we didn't need more witnesses. We needed more senators with spines."

More than 200 people have been charged with federal crimes in the bloody assault on Congress, and the investigations are still ongoing.

(With input from agencies)