WASHINGTON (AFP) - One year after supporters of Mr Donald Trump stormed the US Capitol and shut down Congress, Americans still await a reckoning on the unprecedented challenge to the country's democracy.

Was it a simple protest-turned-riot? An insurrection? A coup attempt plotted by Mr Trump?

Videos from Jan 6, 2021 bear witness to the violence wrought in the former president's name.

Attackers are seen beating security officers with iron bars and clubs. A policeman is crushed in a doorway, howling in pain.

Rioters clad in assault gear chant "Hang Mike Pence," while the former vice-president and Democratic and Republican lawmakers flee. A woman is fatally shot in a Capitol hallway.

Americans were stunned by the hours-long assault, and so was much of the world, accustomed to seeing the United States as a model of stable democracy.

One year later, the brazen attempt to prevent Democrat Joe Biden from taking office after his victory in the November 2020 presidential election needs an accounting.

"Not even during the Civil War did insurrectionists breach our Capitol, the citadel of our democracy," Mr Biden said in July.

"This was not dissent. It was disorder. It posed an existential crisis and a test of whether our democracy could survive."

A year later, more than 700 people involved in the Jan 6 attack have been charged, for assaulting law enforcement officers and breaking into and desecrating the halls of Congress.

Investigations have shown a concerted effort by Mr Trump and his allies to prevent Mr Pence from leading Congress in certifying Mr Biden as the lawfully elected president.

The looming question is: how are the attack and Mr Trump's effort linked?

A special committee of the House of Representatives is investigating, but the deeper they get, the more sensitive it becomes.

If they find evidence suggesting that Mr Trump knowingly incited the attack, or plotted to illegally keep power, should they risk more turmoil by seeking an unprecedented criminal prosecution of an ex-president?

For the first anniversary of the attack on Thursday, House Speaker Nancy Pelosi has ordered a "solemn observance" in Congress.

Mr Trump, who remains the most powerful figure in the Republican Party, plans his own Jan 6 commemoration in Palm Beach, Florida, which he says will focus on the "rigged" presidential election of 2020.

Although he has shown no evidence that the election was fraudulent, polls show that around two-thirds of Republican voters believe him.

And Republican lawmakers, aware that Mr Trump can make or break them politically, have almost uniformly fallen in line.

Even Mr Pence won't speak against him. Instead, the party is seeking to regain power in the 2022 congressional elections and in 2024, when Mr Trump could run again for president.

Concerted effort to nullify election

The arc of events leading to Jan 6, 2021 has become clearer.

Months before the vote, Mr Trump declared it would be fraudulent and he would not accept losing.

On election night when Mr Biden's victory was clear, he refused to concede. For six weeks, Mr Trump and his backers sought to reverse vote counts in key states by lawsuits and pressure on leaders.

When that effort failed, they set their sights on Jan 6, when Mr Pence was to convene the two houses of Congress to certify Mr Biden's victory.

One point of attack was to summon Mr Trump supporters to Washington.

"Big protest in DC on Jan 6th," Mr Trump tweeted. "Be there, will be wild!" Another was to pressure Mr Pence to halt the certification, based on dubious legal justifications mapped out and circulated by Mr Trump allies, his chief of staff Mark Meadows, and some Republican lawmakers.

Those two efforts merged on Jan 6. As Congress prepared to meet, Mr Trump told followers at a White House rally that the election had been "rigged" and vowed to "never concede." Mr Pence was the key, he said.

"If Mike Pence does the right thing, we win the election." He urged the crowd to descend on Congress and to "fight like hell."

Thousands marched to the Capitol, including members of militant groups called the Proud Boys and Oath Keepers, many wearing combat-style body armour and helmets.

In a nearby hotel, Mr Trump allies operated a "war room" in touch with people on the street, with Mr Trump's Oval Office, and with Republican lawmakers in Congress.

The violent attack that came next shut down the Capitol, halting certification. It sent lawmakers fleeing and left five people dead and scores injured.

It took police and federal troops more than six hours to regain control and remove the attackers.

Finally, in the early morning hours of Jan 7, Mr Pence officially certified Mr Biden as president-elect.

Rush to complete probe

Many thought the rapid-fire impeachment of Mr Trump over the following two weeks, and Mr Biden's inauguration on Jan 20, would consign the whole episode to history.

But Mr Trump didn't go away. He secured his power over the Republican Party, rejected all criticism, and pledged a comeback.

Democrats, aghast, are demanding a public reckoning. "Inaction - or just moving on - is simply not an option," said Democratic Congressman Bennie Thompson, who leads the House investigation.

The committee, which has already interviewed around 300 people, needs to complete its work before the November 2022 midterm elections, when Republicans could retake control of the House and snuff out the probe.

In December, committee vice-chair Liz Cheney, one of a handful of Republicans to support the investigation, made clear the panel's sights were on Mr Trump.

"There has been no stronger case in our nation's history for a congressional investigation into the actions of a former president," Ms Cheney said. "We cannot surrender to president Trump's efforts to hide what happened."

Experts say baring the truths about Jan 6 poses huge political risks for the Biden administration. But leaving them buried is also dangerous.

"Jan 6 was the harbinger of a clear and present danger," Mr William Galston, a political scientist at the Brookings Institution, told AFP.

"The effort to nullify the results of a democratic election failed," he said. "Will that be true three years from now? That's not so clear. Because the people who were determined to nullify the effects of the 2020 election have learned a lot."