WASHINGTON — With never-seen video, new audio and a mass of evidence, the House committee investigating the Jan. 6 attack on the U.S. Capitol will attempt to show the “harrowing story” of the deadly violence that erupted that day and also a chilling backstory as the defeated president, Donald Trump, tried to overturn Joe Biden’s election victory.

Thursday’s prime-time hearing will open with eyewitness testimony from the first police officer pummeled in the mob riot and from a documentary filmmaker tracking the extremist Proud Boys, who prepared to fight for Trump immediately after the election, and led the storming of the Capitol.

It will also feature the committee’s accounts from Trump’s aides and family members, interviewed behind closed doors, of the deadly siege that Democrats and others say put U.S. democracy at risk.

“When you hear and understand the wide-reaching conspiracy and the effort to try to corrupt every lever and agency of government involved in this, you know, the hair on the back of your neck should stand up,” Rep. Elaine Luria, D-Va., a member of the 1/6 committee, said in an interview.

“Putting it all together in one place and one coherent narrative, I think, will help the American people understand better what happened on January 6th — and the threats that that could potentially pose in the future.”

The 1/6 panel’s yearlong investigation will begin to show how America’s tradition of a peaceful transfer of presidential power came close to slipping away. It will reconstruct how Trump refused to concede the 2020 election, spread false claims of voter fraud and orchestrated an unprecedented public and private campaign to overturn Biden’s victory.

The result of the coming weeks of public hearings may not change hearts or minds in politically polarized America. But the committee’s investigation with 1,000 interviews is intended to stand as a public record for history. A final report aims to provide an accounting of the most violent attack on the Capitol since the British set fire to it in 1814 and to ensure such an attack never happens again.

Unapologetic, Trump dismisses the investigation as illegitimate — and even declared Thursday on social media that Jan. 6 “represented the greatest movement in the history of our country.”

The riot left more than 100 police officers injured, many beaten and bloodied, as the crowd of pro-Trump rioters, some armed with pipes, bats and bear spray, charged into the Capitol. At least nine people who were there died during and after the rioting, including a woman who was shot and killed by police.

Emotions are still raw at the Capitol, and security will be tight for the hearings. Law enforcement officials are reporting a spike in violent threats against members of Congress.

Against this backdrop, the committee will speak to a divided America, ahead of the fall midterm elections when voters decide which party controls Congress. Most TV networks will carry the hearings live, but Fox News Channel will not.

The committee chairman, civil rights leader Rep. Bennie Thompson, D-Miss., and vice chair Rep. Liz Cheney, R-Wyo., the daughter of former Vice President Dick Cheney, will set the tone with opening remarks.

The two congressional leaders will outline what the committee has learned about the events leading up to that brisk January day when Trump sent his supporters to Congress to “fight like hell” for his presidency as lawmakers undertook the typically routine job of certifying the previous November’s results.

“People are going to have to follow two intersecting streams of events — one will be the attempt to overturn the presidential election, that’s a harrowing story in itself,” Rep. Jamie Raskin, D-Md., a member of the committee, said in an interview.

“The other will be the sequence of events leading up to a violent mob attack on the Capitol to stop the counting of Electoral College votes and block the peaceful balance of power,” he said.

First up will be wrenching accounts from police who engaged in hand-to-hand combat with the mob, with testimony from U.S. Capitol Police Officer Caroline Edwards, who was seriously injured in the attack.

Also appearing Thursday will be documentary maker Nick Quested, who filmed the Proud Boys storming the Capitol — and a pivotal leadership meeting with another extremist group, the Oath Keepers, the night before in nearby parking garage.

Leaders of both groups, along with some members, have since been indicted on rare sedition charges over the military-style attack.

Along with the eyewitness testimony, the panel will unveil multimedia presentations, including previously unreleased video and audio, and a “mountain of evidence,” said a committee aide who insisted on anonymity to preview the hearing. There will be recorded accounts from Trump’s senior aides at the White House, the administration and the campaign, as well as members of Trump’s family, the aide said.

Information from Trump’s daughter Ivanka Trump, who urged her father to call off the rioters, is likely to be shared from her private appearance before the committee.

In the weeks ahead, the panel is expected to detail Trump’s public campaign to “Stop the Steal” and the private pressure he put on the Justice Department to reverse his election loss — despite dozens of failed court cases and his own attorney general attesting there was no fraud on a scale that could have tipped the results in his favor.

The panel, made up of nine lawmakers, faced obstacles from its start. Republicans blocked the formation of an independent body that could have investigated the Jan. 6 assault the way the 9/11 Commission probed the 2001 terror attack.

Instead, House Speaker Nancy Pelosi ushered the creation of the 1/6 panel through Congress over the objections of Senate Republican leader Mitch McConnell. She rejected Republican-appointed lawmakers who had voted on Jan. 6 against certifying the election results. There are seven Democrats and two Republicans.

Many Republicans are defending Trump. Rep. Elise Stefanik of New York said at a GOP leadership news conference that the committee’s “shameless prime-time show” is nothing but a smear campaign against the former president, his party and his supporters.

But by many measures, the attack was set in motion shortly after Election Day, when Trump falsely claimed the voting was rigged and refused to concede once Biden was declared the winner.

The proceedings are expected to introduce Americans to a cast of characters, some well known, others elusive, and to what they said and did as Trump and his allies tried to reverse the election outcome.

The public will learn about the actions of Mark Meadows, the president’s chief of staff, whose 2,000-plus text messages provided the committee with a snapshot of the real-time scramble to keep Trump in office. Of John Eastman, the conservative law professor who was the architect of the unsuccessful scheme to persuade Vice President Mike Pence to halt the certification on Jan. 6. Of the Justice Department officials who threatened to resign rather than go along with Trump’s proposals.

Lawmakers have also been caught up in the probe, including House GOP leader Kevin McCarthy, who defied the committee’s subpoena requests for testimony.

The Justice Department has arrested and charged more than 800 people for the violence that day, the biggest dragnet in its history.

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Associated Press writers Kevin Freking and Michael Balsamo contributed to this report.