The acting head of US Capitol Police apologized to Congress Tuesday for security failures during the Jan. 6 siege, which she characterized as a “terrorist attack” involving tens of thousands of “insurrectionists,” testimony shows.

Yogananda D. Pittman, the agency’s acting chief, offered her “sincerest apologies” on behalf of the department during a closed-door briefing Tuesday before the House Appropriations Committee.

“Let me be clear: the Department should have been more prepared for this attack,” Pittman said, according to a copy of her testimony obtained by The Post.

“We knew that militia group 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.”

Pittman acknowledged the “strong potential” for violence at the Capitol — with Congress as a target — prior to the deadly unrest, noting that department officials knew it would be unlike previous protests as recently as Jan. 4.

“The Department prepared in order to meet these challenges, but we did not do enough,” Pittman’s testimony continued.

Capitol Police changed its operational plan for Jan. 6 two days earlier, with then-head Steven Sund requesting that the Capitol Police Board declare a state of emergency and approve a request for support from the National Guard, Pittman said.

But the board denied the request, according to Pittman’s testimony, instead telling Sund to reach out to the National Guard in Washington, DC, to find out how many Guardsmen could be sent to the Capitol on short notice, “which he did,” Pittman said.

Sund then requested “additional boots on the ground” from federal agencies and other law enforcement entities as the siege began to unfold and wanted approval from the Capitol Police Board to bring in the National Guard, but was not “granted authorization for over an hour,” Pittman said.

The acting chief said more than 1,200 Capitol Police officers were working at the site when the siege unfolded — leaving five people dead, including Capitol Police Officer Brian Sicknick.

But cops were “no match for the tens of thousands of insurrectionists” — many of whom were armed — intent on storming the Capitol and refusing orders to stop, Pittman said.

“In my experience, I do not believe there was any preparations that would have allowed for an open campus in which lawful protesters could exercise their First Amendment right to free speech and at the same time prevented the attack on Capital [sic] grounds that day,” Pittman’s testimony continued.

Pittman said “many” of the agency’s officers are now suffering from PTSD, especially “after the loss of two of our officers directly and indirectly as a result” of the Jan. 6 siege.

Officer Howard Liebengood, 51, died by suicide days after responding to the riots, his family has said. He was a member of the department since 2005, department officials said.

“As the Acting Chief, I take responsibility for the mistakes that were made by the Department, and I pledge to this Committee, the Congress, the American people, and my USCP colleagues, that we will do better going forward, but we need to make changes,” Pittman said.

An in-depth review is underway, but Pittman said the agency immediately needs additional resources in the form of manpower and physical assets to carry out its task.

“We know the eyes of the country and the world are upon us,” Pittman said, according to her prepared remarks. “The US Capitol Police remain steadfast in addressing the new challenges that we face head on.”