U.S. House of Representatives Speaker Nancy Pelosi called on Capitol Police Chief Steven Sund to resign on Thursday, after the federal force charged with protecting Congress was unable to keep rioters from storming the building.

Some officers in the 2,000-member Capitol Police fell back as crowds advanced on Wednesday, enabling an angry mob to invade the halls of Congress and disrupt certification of Democratic President-elect Joe Biden’s victory. Other Capitol officers fought to keep lawmakers and staff safe, Reuters report .

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

The Capitol Police, overwhelmed by the crowds, took responsibility for speeding lawmakers to safety and ejecting those who had gotten inside, according to two people familiar with the incident. Once rioters were outside, D.C. police handled removing them from the external stairs, porticos and balconies of the Capitol.

Very few people were arrested for the breach, one person said, because officers didn’t have enough backup to take the time to arrest and detain them, the Washington Post reports.

Sund defended his agency in a statement.

"United States Capitol Police (USCP) officers and our law enforcement partners responded valiantly when faced with thousands of individuals involved in violent riotous actions as they stormed the United States Capitol Building. These individuals actively attacked United States Capitol Police Officers and other uniformed law enforcement officers with metal pipes, discharged chemical irritants, and took up other weapons against our officers. They were determined to enter into the Capitol Building by causing great damage," the statement reads in part.

Former U.S. Capitol Police Chief Terrance Gainer told NBC he wants to give police "the benefit of the doubt" and hopes they were attempting to de-escalate Wednesday's events when they appeared to let rioters inside the legislative building.

"Sometimes when you don't have enough personnel, you can't stand and fight a large crowd like that," he said on the "Today" show Thursday, noting that there were not enough law enforcement personnel on scene.

Gainer, who served as Capitol Police Chief from 2002 to 2006 and was the Senate sergeant-at-arms, said the police's handling of the situation was "a failure" and "raises a lot of questions."

“It’s like watching a real-life horror movie. I mean, we train and plan and budget every day, basically, to have this not happen,” said Kim Dine, who was chief of the Capitol Police from 2012 to 2016. “How it happened, I can’t figure that out,” he told the Washington Post.

Bill Bratton, the former commissioner of the New York Police Department and an NBC News analyst, said there needs to be an explanation about why the crowd was let in. "Right now the appearance of it looks awful," he said.

Some experts defended the police, noting that the Capitol Police deals with protests, both inside and outside the building, legal and illegal, on a regular basis, but they are normally nothing like Wednesday's riot.

“This isn’t what happens at the U.S. Capitol,” said Chuck Wexler, the executive director of the Police Executive Research Forum. “This is completely unprecedented.”

Wexler told the New York Times that legitimate questions would be raised about why more officers were not on hand and why they did not anticipate the threat.

Chief Michel Moore of the Los Angeles Police Department said it was unrealistic to expect the police to quell “people who are hell bent on destruction.”