With the country still reeling from Wednesday's violence in Washington, serious questions are being asked about how such a massive security breach was able to happen at the heart of US government.

Crowds of pro-Trump supporters were able to force their way inside one of the country's most historically and politically significant buildings while elected lawmakers were inside moving to certify Joe Biden's election victory.

The world watched as a mob of rioters seemed to roam free around inside - looting and vandalising symbols of US democracy as they went.

President-Elect Joe Biden has been scathing of the "unacceptable" handling of the rioters and compared it to the heavy-handed militarised response to last year's Black Lives Matter protests.

Lindsay Graham, a Republican Senator, also railed against the security failures. "They could have blown the building up. They could have killed us all. They could've destroyed the government," he said.

How could this be allowed to happen?
Criticism centres on preparation by police and their failure to anticipate possible violence, despite evidence that radical pro-Trump supporters and other groups were openly discussing their plans online.

The Washington Post, citing sources close to the matter, says that Capitol Police charged with guarding the building and its grounds did not make early requests for help from the city's main police force or the National Guard nor set-up a multiagency command centre to coordinate response to any violence.

And without an adequate security perimeter in place, their sparse police lines were quickly overwhelmed by thousands descending on the Capitol.

Dozens of officers were injured, and one later died, in the effort to retake control - including some with armour, weapons and chemical spray agents.

Did police treat Trump supporters differently?
To many, the optics were a sharp contrast to last year's protests following the death of George Floyd, when rows of National Guard Troops guarded and enforced order in the capital.

Even hours into Wednesday's violence, protesters were filmed being escorted or guided out of the building without arrest - even appearing to be helped down the Capitol stairs and having doors held open for them to exit. Another viral clip appeared to show a police officer posing for a selfie with a man inside.

Many rioters photographed and even live-streamed their crimes. One was pictured, his face uncovered, with his feet up on House Speaker Nancy Pelosi's desk, and then showing off a letter he appeared to have stolen from her office. A Confederate flag was paraded by another unmasked man and a well-known conspiracy theorist - wearing horns, fur and facepaint - was seen posing by a Senate chair that had been occupied by Vice President Mike Pence just hours earlier.

Nick Ochs, a known member of the Proud Boys far-right group, tweeted a selfie of himself inside and later told CNN: "There were thousands of people in there - [the police] had no control of the situation. I didn't get stopped or questioned."

But despite the severity and scale of the chaos, relatively few arrests had been made by nightfall Wednesday.

Supporters of BLM and others on the left have voiced their outrage at the perceived double-standard of policing the events. The incoming vice-president, Kamala Harris, said the disconnect was "unacceptable".

"I cannot think about moving on or turning the page until we reckon with the reality of what we saw yesterday," she wrote. "True progress will be possible only once we acknowledge that this disconnect exists and take steps to repair it."

Top congressional Democrats Chuck Schumer and Nancy Pelosi were damning in their criticism of security failures - helping to force the departures of top officials.

The chief of the US Capitol Police, Steven Sund, had initially defended his force in a statement on Thursday - describing the actions of law enforcement as "heroic" and their plan robust.

But within hours his resignation was announced, alongside the departures of the sergeant-at-arms for both the House and Senate.

Aside from the clear lack of preparation, confusion mounted during the violence about when and if other security forces were being deployed to help.

According to the Washington Post, Pentagon officials had placed strict operational limits on the DC National Guard ahead of protests and remained concerned about the "optics" of armed military personnel at the Capitol.

Defence officials on Thursday sought to defend the speed in which they authorised and mobilised Guardsmen to respond to the violence.

Multiple US media outlets, citing senior sources, have suggested that President Donald Trump allegedly showed reluctance for the National Guard to be used to quell the unrest.

If true, this is a complete contrast to the highly visible show of force the president has repeatedly called for against left-wing and BLM protesters. Gordon Corera, the BBC's security correspondent, says this emphasises how security decisions appear to have become politicised under the Trump administration.

Professor Clifford Stott, a specialist in the policing of crowds who advises the UK government, has been analysing the police response to BLM protests in Seattle. He told the BBC there would be "powerful and important" questions to be asked about how officials failed to prepared for the escalation by Trump supporters.

"It was the failure to predict that that led them to be inadequately prepared when it did happen and led them to be reactive and have to mobilise more resources," he said. "It's not just about the complexities of the police response, it's also about what appears to be a poor level of risk assessments around how they understand whether resources might be necessary in the first place."

What was known in advance of violence?
The gathering of the president's supporters, while Congress was certifying the election result, was not spontaneous. The protest was pre-planned and followed months of escalating rhetoric from President Trump and some of his Republican allies seeking to undermine the result.

In the days (and indeed weeks and months) before the attack, people monitoring online platforms used by extreme pro-Trump supporters and far-right groups had warned of rhetoric encouraging violence at the Capitol, including toward lawmakers, over the election result. Some were even pictured wearing clothing that said "MAGA: CIVIL WAR" printed alongside the 6 January 2021 date.

Prof Stott, who researches the psychology of group violence and hooliganism, told the BBC he found the overarching mood of "joy" among rioters openly committing crimes particularly interesting.

"There was a very clear purpose to that crowd and that was driven by the idea their actions were legitimate, given their perception that their president - as their commander in chief - had sanctioned them to go and do this," he says. "And that sense that Capitol Hill itself had been over taken by corruption."

Some appeared confused and angry at why officers had used force against them. One Yahoo News video, viewed more than 25m times on TikTok, shows a woman visibly upset that she had been maced by police, despite declaring: "We're storming the capitol - it's a revolution!"

Some radical supporters have responded with disbelief and frustration at the concession video shared by President Trump on Thursday following the violence.

The U-turn came only a day after he told supporters: "We will never give up, we will never concede" and openly pressured his vice-president to overturn the election result.

Will the rioters face legal action?
Vice-President Mike Pence is among those who have called for those involved in the breach to be prosecuted to the fullest extent of the law and given the brazen behaviour of many involved, prosecutors surely have no shortage of evidence to draw from.

As Facebook moved to remove videos seen to incite or encourage the events, some open-source investigators called for people to archive evidence to help with crowdsourced identification.

Meanwhile, many seen in viral images from inside are already known figures within far-right groups and QAnon and related conspiracy networks.

Dozens of people are already facing charges - including one man officials say had a semi-automatic rifle and 11 Molotov cocktails - and police have also appealed for help identifying offenders not yet in custody.

Michael Sherwin, the acting US Attorney for DC, said on Thursday that prosecutors would bring "the most maximum charges we can", when asked if crimes such as seditious conspiracy and insurrection could be tabled.

He also refused to rule out investigating anyone deemed to have incited the violence, including President Trump.

"We're trying to deal with the closest alligators to the boat right now," he said.

"Those are the people who obviously breached the Capitol, created violence and mayhem there and then exited. But yes, we are looking at all actors here, not only the people that went into the building."