Capitol Police Chief Steven Sund's resignation was sought by House Speaker Nancy Pelosi after the federal force charged with protecting Congress was unable to keep Trump's supporters from storming the Capitol on Wednesday.

Officers in the 2000-member Capitol Police fell back as crowds advanced on Wednesday, enabling Trump supporters angry about his election defeat to invade the halls of Congress to disrupt certification of Democratic President-elect Joe Biden's victory. Other officers fought to keep lawmakers and staff safe.

Washington's federal prosecutor said he would charge any Capitol police officers found to be complicit in allowing protesters into the building, and lawmakers vowed to open an investigation into the department.

"He hasn't even called us since this happened," Pelosi said of Sund during a news conference on Thursday.

"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 said that Paul Irving, the House sergeant-at-arms, would resign. Top Senate Democrat Chuck Schumer 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 computers and documents, 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 crowd racing through the building - with Trump flags and his signature red hats - stood in sharp contrast to the response to anti-racism protests 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.

District Councilman Charles Allen, who represents the area around Capitol Hill, called that contrast particularly jarring.

"They were overrun and, in many cases, appear to have completely opened the gates, snapped selfies," Allen said. "By the time they called D.C. Metropolitan Police Department, it was too late."

House Appropriations Committee Chairwoman Rosa DeLauro and Representative Tim Ryan, who is leading a probe, said in a statement: "There was a severe systemic failure in securing the building's perimeter and in the response once the building was breached."

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.

The aide said that some protesters were found carrying zip ties. Those heavy-duty plastic ties are often used to handcuff people, and could indicate an intent to kidnap members of Congress.

Pelosi and other lawmakers also praised the Capitol Police for protecting their staff and journalists. Many officers were injured.

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".

Australian Associated Press