In her opening statement, Rep. Liz Cheney, R-Wyo., said the panel's first choice was to have an independent, bipartisan commission not made up of lawmakers to investigate the attack -- but that effort was killed by Republican leadership.

"That leaves us where we are today. We cannot leave the violence of Jan. 6 and its causes uninvestigated," she said. "If those responsible are not held accountable, and if Congress does not act responsibly, this will remain a cancer on our constitutional republic."

Cheney also reminded that Republicans had "recognized the events that day for what they actually were" in the days after the attack, even if members downplay it now, but said the committee's work is just beginning.

"We must issue and enforce subpoenas promptly," she said. "We must overcome the many efforts we are already seeing to cover up and obscure the facts."

She then called out to every member of Congress to ask themselves: "Will we adhere to the rule of law, respect the rulings of our courts, and preserve the peaceful transition of power? Or will we be so blinded by partisanship that we throw away the miracle of America? Do we hate our political adversaries more than we love our country and revere our Constitution?"

She added, "I pray that we all remember, our children are watching, as we carry out the solemn and sacred duty entrusted to us. They will know who stood for truth. They will inherit the nation we hand to them -- a republic, if we can keep it."

At the end of a video with never-before-seen footage of the attack, one rioter said they'll be back, which Thompson said was a warning that "this threat hasn’t gone away" but "looms over our democracy like a dark cloud."

Thompson closed his opening statement by saying while the attack was fueled by a "vile, vile lie," his committee will be a beacon for uncovering the truth of that day.

"The rioters who tried to rob us of our democracy were propelled here by a lie. As Chairman of this Committee, I will not give that lie any fertile ground," he said.

"We cannot allow ourselves to be undone by liars and cheaters. This is the United States of America," he added.

Opening the hearing, Chairman Bennie Thompson, D-Miss., said there's "no place for politics or partisanship" in their investigation and praised the police officers for testifying for the panel.

"For appearing here, and more importantly, for your heroism on Jan. 6, you have the gratitude of this committee and this country. You held the line that day, and I can’t overstate what was on the line: our democracy. You held the line," he said.

"We’re going to revisit some of those moments today, and it won’t be easy," Thompson added. "But history will remember your names and your actions."

Thompson proceeded to play video from Jan. 6 showing the officers defending the Capitol from a violent, pro-Trump mob, intermixed with their pleas to each other over their radios.

"Just describing that attack doesn’t come close to capturing what actually took place that day, so we’re going to see some of what our witnesses saw on Jan. 6," he said.

The House select committee’s first hearing is underway.

Chairman Bennie Thompson, D-Miss., and Rep. Liz Cheney, R-Wyo., will each deliver opening statements ahead of testimony from four police officers who defended the Capitol on Jan. 6.

Cheney will speak in place of Republicans, whose ranking member would typically be given an opportunity to make opening remarks after the committee chair -- but House GOP Leader Kevin McCarthy pulled his members from the panel, leaving only Cheney and Rep. Adam Kinzinger, who took appointments from Speaker Nancy Pelosi.

Tuesday’s hearing is expected to go two to three hours and will feature new video elements from the attack.