The Capitol Police Department was woefully underprepared to handle the riotous mob of Trump supporters on Wednesday — even though Washington, DC authorities had advanced warning that the crowd would be descending on Capitol grounds.

The federal agency, tasked with guarding the area and members of Congress, underestimated the sheer size — and will — of the protesters who smashed their way inside the Capitol building and ran amok for hours.

“Capitol Police did have a plan, but apparently they assumed business as usual,” one law enforcement official told the Washington Post, saying they did not expect the crowd from President Trump’s rally to target the building and push their way in.

“Bottom line, there just wasn’t enough personnel to prevent a mob from pushing in,” the source said.

Some officers were also out sick on Wednesday due to COVID-19 infections or exposures, sources told the newspaper.

On Thursday, red-faced authorities in DC worked to distance themselves from the embattled federal agency, acknowledging that there was advance intelligence that the throng of pro-Trumpers was coming.

“It was the going-in position that it would be somewhat similar to Nov. 14, Dec. 12 where the types of groups that were there,” Army Secretary Ryan McCarthy said at a press conference referring to other pro-Trump rallies in the nation’s capital, “but had no wildest imagination that you could end up breaching the Capitol grounds.”

Questions are now mounting over how the Capitol Police Department — the 2,300-member force with an annual budget of $460 million — failed to thwart the infamous attack Wednesday.

One viral video shows the outmatched officers on the steps of the Capitol building as they appear to allow the rowdy, flag-toting demonstrators to stream onto the property.

Once inside, they were free to roam about, breaking into the Senate chamber and even House Speaker Nancy Pelosi’s office, despite encountering several Capitol officers.

Video posted on Twitter showed one of the officers stopping to snap selfie with a protester — with other cops appearing to be standing among the crowd that infiltrated the building.

Other details have emerged suggesting that Capitol police took a kid-glove approach to protesters as they even fielded questions about where the bathroom was.

“We just need you guys to get out of here safely,” one officer told a man, according to the New York Times.

Another cop was asked why they weren’t forcing the mob out.

“We just got to let them do their thing for now,” the officer said.

Around the same time, a protester shouted “Traitor!” in another officer’s face.

When a man apologized to him for the outburst, the cop responded, “You’re fine,” The Times reported.

Realizing they were overwhelmed, Capitol police requested help from the Metropolitan Police Department at 1 p.m., MPD’s Acting Chief Robert Contee said Thursday.

But by then, “things were already bad at that point,” he said.

Sometime in the afternoon, Capitol police also made an 11th-hour request for 200 members of the National Guard to assist them, defense officials told the Washington Post — a request that could’ve been made before Wednesday.

“There were discussions previously with Capitol police and no requests of DC National Guard were made,” McCarthy said Thursday about the lack of planning.

In contrast, DC Mayor Muriel Bowser asked for the National Guard to backup the MPD in a request last week. Her ask was granted by the Defense Department on Monday.

Capitol Police didn’t respond to a series of questions from The Post about how it handled Wednesday’s protests.

In a lengthy statement, Capitol Police Chief Steven Sund said the agency is “conducting a thorough review of this incident, security planning and policies and procedures.”

“The USCP had a robust plan established to address anticipated First Amendment activities,” Sund said. “But make no mistake – these mass riots were not First Amendment activities; they were criminal riotous behavior.”

He said more than 50 Capitol and MPD officers were injured in the riots, with several Capitol cops hospitalized with serious injuries.

“The actions of the USCP officers were heroic given the situation they faced, and I continue to have tremendous respect in the professionalism and dedication of the women and men of the United States Capitol Police,” Sund said.