WASHINGTON (AP) — The chairman of the House Homeland Security Committee, Bennie Thompson, didn’t realize the severity of the Jan. 6 insurrection until his wife called him.

He was inside the Capitol, sitting in the upper gallery of the House, hoping for what he called a “bird's-eye view of the process” and to be able to tell his grandchildren that he was there when Congress certified Joe Biden’s presidential victory.

People are breaking into the building, London Thompson told him, and it was on television. “I’m watching people climbing over the wall right now,” she said.

“It doesn’t register,” the Mississippi Democrat recalled in an interview with The Associated Press. “I said, ‘You can’t break in. There’s police and barricades and a lot of things out there.’”

But it was not long before the House chamber was under siege. Police rushed Thompson and several dozen other members of Congress to another side of the gallery and told them to duck under their seats as supporters of then-President Donald Trump tried to break down the doors to the chamber below.

“It was a horrible day,” said Thompson, "still almost surreal that it even occurred."

Like Thompson, many who serve and work in the Capitol are trying to make sense of the chaos that unfolded on Jan. 6. And he now has a guiding role in the process, appointed by House Speaker Nancy Pelosi, D-Calif., as the chairman of a select committee that will investigate the attack. The panel will hold its first hearing Tuesday with police officers who battled the rioters.

As the longtime chairman of the Homeland Security Committee, Thompson is accustomed to dealing with grave matters of national security. But his stewardship of the Jan. 6 panel will be a test unlike any other, as he tries to untangle the events of a violent insurrection that many House Republicans increasingly play down and deny.

“We have to get it right,” Thompson said. If the committee can find ways to prevent anything like it from happening again, “then I would have made what I think is the most valuable contribution to this great democracy."

Thompson, 73, is a liberal fixture in Congress and longtime champion of civil rights, the only Democrat in the Mississippi delegation, hailing from a majority-black district in the state’s western half. He has avoided the limelight during his more than 15 years on the Homeland Security Committee, notching achievements with careful bipartisan outreach.

Several Democrats and Republicans said Thompson was the right choice to lead an investigation that is certain to be partisan and fraught.

“I’ve dealt with Bennie for 15 years, and we disagreed on a lot, but I don’t think there was ever a harsh word between us,” says former Republican Rep. Pete King of New York, who was the chairman and top Republican on the Homeland Security Committee for years opposite Thompson. “Bennie is low key, he manages his side well. He was a good guy to work with. He was strong and knew what he wanted, but there was very little drama.”

New York Rep. John Katko, who is now the top Republican on the Homeland Security Committee, gave a similar assessment. Thompson is “a good man, a patriotic American” and a “productive partner,” Katko said in statement.

Pelosi chose Thompson as chairman after he crafted legislation with Katko that would have created an independent, bipartisan commission to investigate the Jan. 6 attack. That bill won almost three dozen Republican votes in the House only to flame out in the Senate, where the opposition of Senate GOP leader Mitch McConnell was decisive.

Far fewer House Republicans supported creating the House select committee, dismissing the effort as partisan. House Republican Leader Kevin McCarthy said the GOP won't participate after Pelosi rejected two of his appointments, Republican Reps. Jim Banks of Indiana and Jim Jordan of Ohio.

Only two Republicans voted to create the panel — Wyoming Rep. Liz Cheney and Illinois Rep. Adam Kinzinger. Pelosi first appointed Cheney to the committee and then added Kinzinger on Sunday after McCarthy withdrew his picks.

“I’m looking forward. in the long run, to try to have as many of the 13 members that I can,” Thompson said last week.

House Intelligence Committee Chairman Adam Schiff, who was appointed to the Jan. 6 committee, said Thompson’s history of working with Republicans and his popularity among members will make it harder to malign the panel’s work. Reaching the bipartisan deal with Katko was not an easy task, he said.

“I think he has a very even keel that will help him get through this,” Schiff said.

Maryland Rep. Jamie Raskin, another Democrat appointed to the select committee, says both parties have “partisan brawlers” — and Thompson is not one of them.

“He’s a workhorse, so he likes getting stuff done,” Raskin said. “And I think that’s the right spirit for this.”

Still, Thompson has taken sharply partisan stances. He joined with about 30 Democrats in a 2005 vote to invalidate President George W. Bush’s victory — not unlike the dozens of Republicans who voted to invalidate Biden’s in January. In that challenge, the dissenting Democrats claimed irregularities if not fraud in Ohio’s vote.

The effort did not end in violence and John Kerry, the defeated Democratic presidential candidate, did not lead or join the effort to deny Bush his victory.

A frequent critic of Trump, Thompson joined other Democrats in filing a lawsuit against the former president after the insurrection, charging that he incited the attack and conspired to prevent Congress from certifying Biden’s victory.

Last week, Thompson withdrew his participation in that lawsuit, which he joined soon after the Senate acquitted Trump, at his second impeachment trial, of inciting the insurrection. Thompson's withdrawal petition said he “wishes to avoid even the appearance of a conflict of interest between his role on the Select Committee and his role as a Plaintiff in this litigation.”

The lawsuit, which is still active, names as defendants Rudy Giuliani, Trump’s personal lawyer, and the Proud Boys and the Oath Keepers. The Justice Department has filed charges against members of those groups in connection to the attack, and the panel is expected to investigate them as part of its probe.

Domestic extremism and its links to white supremacy are a familiar subject for Thompson not only from his time on the Homeland Security Committee but also from his early involvement in the civil rights movement in Mississippi. He was active with the Student Nonviolent Coordinating Committee in college and organized voter registration drives before he was elected mayor of his small hometown of Bolton.

The FBI’s assessments about the growing dangers of domestic extremism, he said, show that “the significance of this committee’s work is as important as it can ever get.”

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Associated Press video producer Padmananda Rama contributed to this report.