WASHINGTON >> The Capitol Police Department today delivered to Congress its first after-action report since the deadly pro-Trump assault on the Capitol, laying out a dismal picture of failure to prepare adequately despite knowing days ahead of time that right-wing extremists could target lawmakers.

Testifying by videoconference to a closed session of a House committee, the acting chief of the Capitol Police, Yogananda D. Pittman, acknowledged that the department knew there was a “strong potential for violence” but failed to take necessary steps to prevent what she described as a “terrorist attack.”

Officers were outmanned, had poor communications, lacked sufficient supplies and struggled to carry out orders like locking down the building, she said.

Her comments were the fullest account to date from the department about its preparations for and response to the events of Jan. 6, when thousands of angry protesters, believing false claims that the election had been stolen, marched on the Capitol, urged on by President Donald Trump.

Pittman, who became acting chief after the riot, told members of the House Appropriations Committee, which oversees funding for the agency, that the Capitol Police “failed to meet its own high standards as well as yours.” She added, “I am here to offer my sincerest apologies on behalf of the department.”

Her testimony also marked the beginning of what is likely to be a series of hearings investigating the law enforcement failures that allowed the building to be occupied for the first time since the War of 1812.

“By Jan. 4., the department knew that the Jan. 6 event would not be like any of the previous protests held in 2020,” Pittman testified. “We knew that militia groups and white supremacists organizations would be attending. We also knew that some of these participants were intending to bring firearms and other weapons to the event. We knew that there was a strong potential for violence and that Congress was the target.”

But when the protest grew violent, the department was hampered by a series of problems, she said.

Just as the mob around the Capitol was swelling, the department had to divert some officers away from the building to respond to the discovery of pipe bombs at the nearby offices of the Republican National Committee and the Democratic National Committee — live devices that would have exploded had they not been defused.

The department was also badly outmanned, given the size of the crowd that converged on the Capitol, she said. Pittman said the Capitol Police had 1,200 people working on site when the attack occurred — with only 170 officers equipped with riot gear — which was “no match” for what she called “the tens of thousands of insurrectionists” in Washington that day, hundreds of whom would later enter the Capitol building.

Among the biggest problems, she suggested, was a delayed response to a plea from her predecessor, Steven Sund, who was chief on the day of the attack and resigned in its aftermath, for the deployment of National Guard troops to help.

Two days before the attack, Sund asked that the Capitol Police Board declare a state of emergency and authorize a request to secure National Guard support. The board denied the request, according to Pittman’s testimony, but encouraged Sund to contact the National Guard to determine how many members could be sent to the Capitol on short notice, which he did.

As the crowd became an increasing threat to the Capitol on Jan. 6, Sund asked for more help from federal agencies and law enforcement agencies in the area. “He also lobbied the board for authorization to bring in the National Guard, but he was not granted authorization for over an hour,” Pittman said.

According to a timeline the Defense Department provided to the committee, Mayor Muriel E. Bowser sought help from the Army at 1:34 p.m., and Sund called for the D.C. National Guard at 1:49 p.m. But the Army did not approve deploying the Guard until 3 p.m. and its members did not arrive at the Capitol to help until 5:40 p.m., more than four hours after Bowser’s plea.

Two of the members of the Capitol Police Board at the time of the attack have already resigned: Paul D. Irving, the House sergeant-at-arms, and Michael C. Stenger, the Senate sergeant-at-arms. The third member, J. Brett Blanton, the architect of the Capitol, is still on the board. Blanton said in a statement that he was not included in any discussions about deploying the National Guard.

During the hearing, the commander of the D.C. National Guard told committee members that his authority to quickly deploy the Guard was removed before the Jan. 6 event.

Maj. Gen. William J. Walker, the commanding general of the District of Columbia National Guard, said he had such authority for the Fourth of July, but the Pentagon required additional approval for a request for the Guard during the Jan. 6 attack, according to three sources familiar with his testimony.

Lawmakers, some of whom were trapped on the House floor and in the House gallery as the mob tried to break into the chamber, have been furious with the security lapse.

“Today, we learned that the attack on the People’s House was preventable and that the government agencies tasked with keeping our country safe failed in their most basic responsibilities,” said Rep. Rosa DeLauro, D-Conn. and the chairwoman of the Appropriations Committee. “To comprehensively protect the seat of government, meaningful reforms are critical. We will continue to pursue answers to the questions that we did not cover today.”

In the wake of the attack, the FBI has arrested more than 130 people, including members of the Oath Keepers and the Proud Boys, far-right extremist groups.

In the 20 days since rioters stormed the Capitol, the FBI has received over 200,000 digital media tips and has opened cases on over 400 suspects, while the U.S. attorney’s office in Washington has charged over 150 criminal cases, most of which began as misdemeanors and have been elevated to felonies.

But the manhunt and investigation is expected to “reach a period of a plateau,” said Michael R. Sherwin, the U.S. attorney in Washington. He said investigators were shifting from identifying and rounding up the most easily identifiable individuals to putting together more complicated conspiracy cases related to possible coordination among militia groups and individuals from different states.

“We have to have the proper evidence to charge these, and we’re going to get it,” said Steven D’Antuono, the FBI assistant deputy in charge of the Washington field office. “All these cases are not based upon social media, and Twitter and Instagram posts. We also have traditional law enforcement tools we need to use — grand jury subpoenas search warrants — and you don’t get that overnight.”

Investigators said that some of those efforts could result in more s

erious cases being charged as soon as this weekend and through the coming weeks.

During the briefing, the acting House sergeant-at-arms, Timothy

P. Blodgett, also said it was “clear there was a failure of preparation,” citing poor communications and a weak perimeter defense of the Capitol.

In an interview, Sen. Christopher S. Murphy, D-Conn., who also sits on a subcommittee with oversight over the Capitol Police, said too many people needed to sign off on critical security decisions.

“There is a structure-of-command problem that has to be solved,” he said. Murphy also questioned whether the architect of the Capitol should be involved in security-related decisions and why he was on the board, which was established in 1873.

Murphy said he had spoken to Sund both on the night of the attack and the day after. At the time, Sund was “vigorously defending” the decision to protect the Capitol with only police forces.

“Sund told us they had no intel to predict what happened and reaffirmed that the next day,” Murphy said.