The hunt began even as the smoke was still clearing on January 6: dozens of federal agents went to work sifting through social media posts, analysing scores of videos and analysing anonymous tips as they scrambled to understand who did what that day in the temple of American democracy.

The effort represents “one of the largest investigations the FBI has ever conducted,” said Lorenzo Vidino, director of the Program on Extremism at George Washington University.

These extraordinary efforts have resulted, in less than a year, in the arrest and charging of more than 725 Donald Trump supporters who, after listening to the outgoing president repeatedly denounce what he claimed was a stolen election, stormed into the Capitol building as members of Congress were poised to certify the victory of Joe Biden.

That list has grown almost by the day, and it could ultimately double in length: while federal investigators originally estimated that 800 people took part in the siege of the Capitol, they now say the number is closer to 2,000.

The accused are predominantly men (87 per cent), most of them white, and with an average age of 39 – “which is generally not the age of extremists,” according to Vidino, whose center has compiled detailed data on those charged.

They come from across the US, with varied socioeconomic profiles (including lawyers, landscapers, real estate agents); and those with military backgrounds or who have faced bankruptcy are significantly overrepresented.

The varied group includes far-right extremists and the conspiracy-minded, but also ordinary supporters of Trump convinced by his insistent claims that the election was stolen.

Most of the accused are not charged with any violence or vandalism but merely with having illegally entered the building; they generally face only misdemeanor charges of trespassing or disorderly conduct on restricted grounds.

Prosecutors appear eager to process members of the group as quickly as possible, often through plea bargain agreements that avoid the need for trial: 165 of the accused have already reached such agreements, and some 50 have been sentenced.

Most of those sentences have been relatively light: one young man, who admitted having stolen a beer from the office of House speaker Nancy Pelosi, was sentenced to 20 days in prison, to be served on weekends – allowing him to keep his job.

But 34-year-old Jacob Chansley, who became instantly famous after pictures of him standing shirtless and wearing a horned fur hat inside the Capitol circulated worldwide, received a more serious sentence of 41 months.

His lawyer Al Watkins says the sentencing gap sends the wrong message. “That perspective does not look right for those who believe that they are political prisoners,” he told AFP.

The longer sentences are just beginning to be handed down, against those accused of the most serious crimes: the approximately 225 individuals accused of acts of violence, notably against Capitol police.

The heaviest sentence so far has gone to Robert Palmer, a 54-year-old Florida man accused of attacking police with boards and a fire extinguisher. He received a five-year sentence.

Some 40 people are being charged with criminal conspiracy, which implies a pre-organised attack.

This serious charge has been levied primarily against members of far-right groups like the Proud Boys, the Oath Keepers and the Three Percenters.

These defendants, some of whom have been held in preventive detention for months, are expected to face jury trials beginning as soon as February.

One member of the Proud Boys, a New York man in his 30s, has agreed to cooperate with prosecutors in exchange for a lighter sentence.

So far, no one is being accused of sedition or insurrection – serious charges mentioned early in the investigation but which are difficult to prove.

Vidino said prosecutors have been “trying to be as imaginative as possible” in framing charges. In the US, he noted, investigators can pursue foreign extremist groups but not American organisations that may have radical or violent ideologies.

A key question remains: Who, among key figures not at the Capitol building that day, may have incited or orchestrated the assault. For now, investigators are leaving that matter up to the members of Congress pursuing their own investigation.

Even if Republican senators saved Trump from conviction in his impeachment trial in February, he is not yet in the clear.

The House of Representatives created a select committee to cast light on the role of the former president and his advisors. If it finds sufficient grounds, nothing would prevent prosecutors from filing charges.

And that would open a weighty new chapter in the already sprawling investigation.