WASHINGTON - Deep inside the US Capitol, gunshots rang outside the House of Representatives as a handful of security officers inside scrambled to pile furniture against the doors to keep out the surging crowd.

Inside, congressmen and congresswomen cowered, struggling to put on gas masks.

It was unprecedented for the heart of American democracy. And questions are mounting over the security failure that allowed thousands of Donald Trump supporters to storm the building on Wednesday (Jan 6), with hundreds forcing their way in, smashing windows and vandalising offices.

An unknown number of papers and at least one lawmaker's laptop were stolen as the crowd rampaged through offices. One person walked off proudly carrying Speaker Nancy Pelosi's lectern.

News photographers were assaulted and "Murder the media" was scrawled on a door in the building. On Thursday, Capitol Police chief Steven Sund resigned, as did the Sergeants-at-Arms for the Senate and House of Representatives.

Even with advance news of the rally and open chatter in online forums that the Capitol was a target, security was far lighter than at events such as the Black Lives Matter protests last summer. The contrast and irony were hard to miss.

"We spend US$750 billion (S$994 billion) on defence, and the Pentagon, and we… couldn't control an insurrection in the US Capitol," Democratic congresswoman Pramila Jayapal said on Thursday.

As many as 60 Capitol Police officers were injured in the mayhem, many hit on the head with lead pipes. At least 15 had to be hospitalised. On Thursday, one died.

Capitol Police are some 2,000-strong, but without additional crowd-control resources, they were utterly overwhelmed.

Without back-up, the only way to stop a crowd of thousands would have been to shoot at them - so the police, while resisting to the point of hand-to-hand fighting in the halls and corridors of the hallowed building, had to give way.

Still, one Trump supporter, a woman identified as Ashli Babbitt, was fatally shot by a Capitol Police officer. Another woman and two men died during the mayhem because of heart attacks and strokes.

But some videos circulating online appear to show one Capitol Police officer taking selfies with the mob. And a Washington DC Metro police officer in a public Facebook post wrote that among the rioters were off-duty police officers and members of the military who flashed their badges and identity cards as they overran the building.

Adding to the concern, on Thursday, Pentagon officials said Capitol Police earlier turned down an offer of additional National Guard troops. Reinforcements arrived only after 2pm, after Acting Defence Secretary Chris Miller called up 1,100 members of the DC National Guard. Later, around 6pm, the governor of neighbouring Virginia rushed in additional National Guard troops.

Even Republican Senator Lindsey Graham, a Trump ally, noted that there was better protection in place during Black Lives Matter marches last year.

"Anyone in charge of defending the Capitol failed their duties. If they would have been in the military, they would have been relieved of their commands and most likely court-martialled," Mr Graham said.

President-elect Joe Biden, who spent 36 years in the Senate, said in a video statement: "No one can tell me that if it had been a group of Black Lives Matter protesting yesterday, they wouldn't have been treated very, very differently than the mob of thugs that stormed the Capitol. We all know that's true, and it is unacceptable."

Photographs of the pro-Trump mob reveal several who seemed to have come with clear intent. One revealed a man with plastic ties of the sort police use to handcuff people. Capitol Police did make 14 arrests, including one of a 70-year-old man from Alabama who they said had a gun and materials for Molotov cocktails.

Arizona Congressman Ruben Gallego, a Democrat, told Politico: "If this were an organised, fully intent terrorist group - and there were certainly terrorist activities yesterday, but I mean, al Qaeda style - they could have killed a lot of representatives and god knows what else."

Mr Malcolm Nance, a national security analyst, told radio WBUR that Capitol Police should be investigated for potential complicity. "We saw this coming a long time ago," he said.