Officers in the 2,000-member Capitol Police fell back as crowds advanced on Wednesday, allowing Trump supporters angry about his election defeat into the halls of Congress to disrupt certification of Democratic President-elect Joe Biden's victory. Other officers fought to keep lawmakers and staff safe.
"He hasn't even called us since this happened," Pelosi said of Sund during a news conference.
"Many of our Capitol Police just acted so bravely and with such concern for the staff, the members, for the Capitol ... and they deserve our gratitude. But there was a failure at the top of the Capitol Police," said Pelosi, a Democrat.
She also announced that Paul Irving, the House sergeant-at-arms, would resign his position. Senator Chuck Schumer, the Senate's top Democrat, said he would fire Michael Stenger, the Senate sergeant-at-arms, as soon as runoff elections in Georgia for two seats are certified and Democrats control the Senate.
Trump supporters ransacked legislators' offices, stole at least one computer and left threatening messages as they roamed the building for hours in a rampage that left four people dead: one shot by police and three who died of medical emergencies.
The images of the crowd racing through the building - many with Trump flags and his signature red hats - stood in sharp contrast to the handling of anti-racism protests near the White House this summer.
At that time, the White House was surrounded by multiple blocks of buffer, and law enforcement officers used tear gas, projectiles and at one point the downward blast of a helicopter rotor to push back protesters.
In comparison, the streets around the Capitol were open on Wednesday morning, and scatterings of Capitol Police stood at low metal barriers resembling bicycle racks.
"There was a severe systemic failure in securing the building's perimeter and in the response once the building was breached," House Appropriations Committee Chairwoman Rosa DeLauro and Representative Tim Ryan, who is leading a probe, said in a statement.
A House Democratic aide, speaking on condition of anonymity, said the investigation would focus on what intelligence was available in the run-up to the riot, how decisions were made and why the perimeter of the Capitol was not more secure.
When she joined dozens of House members in the safe room where they sheltered after protesters battered the doors of their chamber, Pelosi vowed to review how long it had taken reinforcements for the Capitol Police to arrive.
Senator Chris Murphy, the top Democrat on the Senate subcommittee overseeing the Capitol Police, said he expected the Senate would coordinate any investigation with the House.
"Taxpayers deserve to know why ... it took only an hour for a handful of ragamuffin protesters to enter the building and pose a threat to the continuity of democracy," he told reporters. "Obviously, we are going to have to look at the way we spend money on the Capitol Police."
'WHY ARE THEY NOT COMING?'
Pelosi recalled a steady stream of calls asking why reinforcements had not been dispatched more quickly.
"Why are they not coming now? Almost a constant flow of calls," Pelosi said. "That is something to review."
Pelosi and other lawmakers also praised the Capitol Police for protecting, their staff and journalists. Many officers were injured.
Schumer told Politico he would fire Stenger when his party takes the majority this month if Stenger has not quit by then.
Republican Senate Leader Mitch McConnell called the breach a "shocking failure."
Capitol Police did not respond to inquiries about why they had not cordoned off the area or brought in more support officers ahead of the protests, which Trump himself urged to be "wild."
The acting US attorney for Washington, Michael Sherwin, told a news briefing that any Capitol Police officer found complicit in the breach of the building would be charged. He also said one man was arrested with a military-style gun and that Molotov cocktails near the Capitol and sedition charges would be considered in arrests related to the attack.
The FBI sought the public's help in identifying the rioters. Some of the 68 people arrested after Wednesday's incidents were due in court on Thursday.
On Thursday, crews installed seven-foot (two-meter) fencing around the Capitol. Police said they had been instructed not to talk to reporters.
Sund in a statement said the officer who fatally shot Ashli Babbitt, who was among a group of Trump supporters trying to force their way into the House chamber, was on administrative leave while the shooting is investigated.
Sund said his officers "responded valiantly" when demonstrators attacked them with "metal pipes, discharged chemical irritants and took up other weapons." They also faced two pipe bombs.
Sund said police would review security plans for Biden's inauguration on Jan. 20.
A senior administration official said the Capitol Police had rejected Defense Department offers of additional training from before Wednesday."There was concern about having too much of a militarized presence, but this was about training and how to set up the perimeters and a layered defense, and they turned that down too," said the official, who requested anonymity.