Law enforcement officials responsible for US Capitol security on January 6 said a violent mob was able to overrun the building because of intelligence failures, a Senate hearing heard.
And those failings were compounded by bureaucratic delays in bringing in National Guard reinforcements, they said.
Former US Capitol Police chief Steven Sund, former Senate sergeant-at-arms Michael Stenger, former House sergeant-at-arms Paul Irving and acting Washington Police chief Robert Contee appeared at the Senate hearing on Tuesday.
They all said their planning was based on intelligence that predicted civil disobedience during a rally of former president Donald Trump’s supporters.
They were not prepared for the protest to turn into a deadly insurrection.
“A clear lack of accurate and complete intelligence across several federal agencies contributed to this event, and not poor planning by United States Capitol Police,” Mr Sund said.
He said his force’s preparations responded to the information they had before time.
“These criminals came prepared for war,” Mr Sund said.
Mr Irving said while they were told that Congress would be a target and some protesters planned to come armed.
“[But] the intelligence was not that there would be a co-ordinated assault on the Capitol, nor was that contemplated in any of the inter-agency discussions that I attended in the days before the attack."
Mr Contee said that as conditions deteriorated around the building, he was “stunned at the response from the Department of the Army, which was reluctant to send the DC National Guard to the Capitol".
Mr Sund and Mr Irving differed in their assessments of the preparation for and response to the riot, especially in a request for National Guard officers, which the Capitol Police chief said was denied two days before the riot.
They also offered different accounts of the timing of events as the attack was under way.
The joint hearing of the Senate homeland security and rules and administration committees is the first in a series of inquiries.
They will look into the origins of the January 6 insurrection and the failure of Capitol security troops to anticipate and then quell the riot, which left five people dead.
Rules chairwoman Amy Klobuchar said senators were planning another hearing next week with Pentagon officials.
All of Tuesday’s witnesses said they were caught off guard by the co-ordinated violence.
The insurrectionists chanted support for Mr Trump and made threats of violence against members of Congress.
The witnesses said they did not have the manpower to deal with such an unprecedented event.
“These resources were barely enough to counter an event that had never happened in the history of the United States: a mob of thousands of American citizens launching a violent assault on the US Capitol – the seat of our government – in an attempt to halt the counting of the electoral ballots," Mr Contee said.
Investigating security flaws is crucial for the politicians, who were the targets of the mob that sought to disrupt a joint session of Congress meeting to certify the presidential election results.
“This hearing is unique because it’s personal to everyone involved,” Gary Peters, chair of the homeland security panel, said in his opening statement.
Ms Klobuchar said: "The stakes are so high. We want this to be as constructive as possible. We must have the facts and the answers are in this room."
She also noted the bipartisan nature of Tuesday’s hearing.
Rob Portman, the top Republican on the homeland security committee, said he wanted answers as to whether US Capitol Police requested National Guard help before January 6.
“And if so, why was that request denied?” Mr Portman asked.
He also wanted to know whether National Guard assistance on that day was delayed, “and why, if that was true".
The assault on the Capitol was replayed in vivid detail at Mr Trump’s Senate trial after he was impeached on a charge that he incited the mob.
The Senate voted February 13 to acquit the former president, but the bitter partisanship has not dissipated, even as members of both parties call for a thorough examination of the riot.
Two House hearings related to the riot are also scheduled this week and retired Army Lt Gen Russel Honore is conducting an independent security review commissioned by House Speaker Nancy Pelosi.
Ms Pelosi also is proposing an independent commission, modelled on the panel that investigated the September 11 terrorist attacks, to review the causes and security failings of the Capitol assault.
One of the main disagreements among the witnesses at Tuesday’s hearing was when Mr Sund asked Mr Irving to call in help from the National Guard.
Mr Sund said he “notified the two sergeants-at-arms by 1.09pm that I urgently needed support and asked them to declare a state of emergency and authorise the National Guard".
He said Mr Irving responded that he “needed to run it up the chain of command”, including congressional leadership.
Mr Irving offered a different version of events.
“I have no memory of a call at 1.09pm and I have reviewed my phone records," he said. "There is no call from Chief Sund or any other person at that time.
"The first call from Chief Sund in the 1.00 hour is at 1.28pm."
Mr Irving said Mr Sund told him that he “might need to make a request for the National Guard”.
Mr Irving denied reports that he was concerned about appearances in preparations for January 6 and calling armed reinforcements as the attack was under way.
"'Optics’ as portrayed in the media did not determine our security posture," he said.
"Safety was always paramount when evaluating security for January 6.
“We did discuss whether the intelligence warranted having troops at the Capitol and our collective judgment at that time was no, the intelligence did not warrant that.”
Mr Contee said he was shocked by the response when the request was finally sent to the Army secretary, which is required for sending in the National Guard in the District of Columbia.
“While I certainly understand the importance of planning and public perception – the factors cited by the staff on the call – these issues become secondary when you are watching your employees, vastly outnumbered by a mob, being physically assaulted,” he said.
“I was able to quickly deploy [the police department] and issue directives to them while they were in the field, and I was honestly shocked that the National Guard could not, or would not, do the same.”
As politicians investigate broader security questions and make changes to prevent future attacks, any disagreement between the officials testifying on Tuesday could add to partisan versions of who is to blame.
Democrats and Republicans are haggling over the composition of the independent commission that Ms Pelosi has proposed.
Republicans have objected to a Democratic proposal for an 11-seat panel that would give seven seats to Democratic appointments, including three to be named by President Joe Biden, and four to Republicans.
Republican leaders want the panel evenly divided.
