The Defense Department is facing scrutiny over its role in the events at the Capitol last week, after the D.C. government and Capitol Police accused Pentagon officials of slow-walking an emergency call for National Guard reinforcements as rioters threatened to breach the building.

The search for answers about why the security breakdown took place, leaving the Capitol all but defenseless against a marauding group of pro-Trump rioters, has taken on increased urgency as Washington prepares for the Jan. 20 inauguration of President-elect Joe Biden and possible new threats from extremists trying to keep President Trump in office despite his loss.

Up to 15,000 National Guard members could be deployed in Washington during the inauguration, senior defense officials said Monday, part of a rapidly expanding response following the deadly insurrection.

The unprecedented breach last Wednesday has been followed by a descent into bureaucratic finger pointing and blame shifting, as local and congressional police have accused the Defense Department of declining to send in Washington, D.C., guardsmen in fast enough, while Pentagon officials have blamed city and congressional police for failing to prepare and request sufficient military help in advance.

The Defense Department has said the Capitol Police, the law enforcement body that protects the U.S. Capitol and reports to Congress, declined multiple offers of help from the D.C. Guard before Jan. 6, and only called with an emergency request after it was too late to get part-time soldiers capable of handling a riot to reach the scene urgently. The Pentagon has also underscored that federal and local law enforcement, not the Defense Department, are responsible for warning about urgent domestic threats.

Steven Sund, who stepped down in recent days from his post as chief of the Capitol Police, said in an interview with The Washington Post that he personally didn’t have any knowledge of outreach to the Capitol Police from the Pentagon offering help from the D.C. Guard ahead of the planned Jan. 6 protests. But a civilian Pentagon official said in a Jan. 3 email obtained by The Post that a U.S. Capitol Police official had circled back and articulated “no requests for DOD support.”

“The Department of Defense approved every request for assistance it received regarding the mob riot at the Capitol,” Pentagon spokesman Jonathan Rath Hoffman said. “And we approved every offer of assistance from others.”

Criticism of the Pentagon by the D.C. and Capitol Police officials has centered on a phone call around 2:30 p.m. on Wednesday, when Army Staff Director Lt. Gen. Walter E. Piatt allegedly told a pleading Sund that there were concerns about sending the D.C. Guard into the Capitol in response to what was a mounting emergency.

Sund recalled Piatt saying: “I don’t like the visual of the National Guard standing a police line with the Capitol in the background.”

City officials said they were flabbergasted by what they interpreted as Piatt fending off their urgent request, even as lawmakers and staff fled a pro-Trump mob that had fought its way into the Capitol building.

Piatt, in a statement issued Monday, denied making such a comment and said he never declined to deploy the D.C. Guard. The three-star general said Army Secretary Ryan C. McCarthy left the room to obtain approval for the Guard’s full activation from acting defense secretary Christopher C. Miller as soon as McCarthy heard the urgent request from the Capitol Police. The D.C. Guard technically answers to the president because the District is not a state, but the Army and defense secretaries oversee the force on the president’s behalf.

“While the two secretaries were meeting, I continued with the call and made clear to the participants of the conference call that I was not the approval authority but that Secretary McCarthy was working the approval,” Piatt said. “I told the assembled group on the call that we need to work together to develop a plan on how to use National Guard Soldiers if their participation was approved.”

Piatt said those plans included the option of dispatching members of the D.C. Guard to free up local D.C. police, who could then assist Capitol Police in fending off the attack on Congress. It also included using the National Guard to set a perimeter at the Capitol so that law enforcement could conduct clearing operations, Piatt said.

“It’s important that in the midst of a dire situation we have a clear plan and understand the task, purpose, and role of our Guardsman before we employ them,” Piatt said. “Creating shared understanding will prevent a complex and potentially dangerous situation from getting worse.”

Piatt said the approval to activate the full D.C. Guard came about 40 minutes after the call.

But the Guard didn’t arrive until later that evening to help establish a perimeter, after D.C. police and federal law enforcement helped the Capitol Police clear the rioters out of the building.

In a recent call, McCarthy told Rep. Jason Crow (D-Colo.), a former U.S. Army Ranger, that because local and congressional authorities didn’t articulate additional needs ahead of the event, the D.C. Guard was not prepared for other contingencies, such as the need to respond swiftly to an insurrection at the Capitol, according to notes on the call that Crow released Sunday.

McCarthy also said that due to a lack of coordination and preparation, there wasn’t a functioning operations center at the Pentagon to manage the small Guard presence that was on the streets of the District on Wednesday and direct additional resources if needed, according to Crow. On Monday, a defense official said a separate state operations center was not required for the limited mission D.C. authorities had requested.

“Substantively, you know, by 8:00 a.m. Wednesday morning, it was largely too late,” Crow said in an interview with The Post. “You can’t mobilize effectively a large force in less than eight hours. Because you have to call people in, and you have to equip them, you have to get them into the armory, you’ve got to transport them, you’ve got to brief them.”

The Pentagon had massed a small quick-reaction force comprised of 40 D.C. Guard members at nearby Andrews Air Force Base to send out in the event that the Metropolitan Police needed additional help beyond the few hundred guardsmen provided for traffic control. But that group didn’t immediately respond when the rioters entered the Capitol, because no contingency operations had been planned with the Capitol Police.

By the morning of Jan. 6, “a lot of the die was cast,” Crow said. “The opportunity had been missed. The planning that would have had to have occurred to respond in the way that we needed to respond, that window had already passed.”

Critics of the Pentagon have said defense officials were happy to carve out a narrow role for the National Guard ahead of the events, and didn’t push for additional involvement or contingency planning, in part because they wanted to keep the military out of the political fray and avoid the backlash the Defense Department faced last summer.

In June, Trump strong-armed city officials and militarized Washington in response to racial justice protests and looting, flooding the streets with Guardsmen and unmarked federal agents and threatening to invoke the Insurrection Act to bring in active-duty forces — and proving how aggressively the federal government could respond to a perceived threat without city sign-off.

The president made no such demands ahead of the planned protests by his supporters on Jan. 6. According to defense officials, Trump told Miller and Gen. Mark A. Milley, chairman of the Joint Chiefs of Staff, at a meeting three days before the event that they should activate the D.C. Guard as they see fit. One of the officials said the president didn’t interact with Miller regarding the response on Jan. 6 itself.

The blowback the military received last June overshadowed officials’ approach to last week’s planned protests, because the Pentagon came under intense scrutiny for its involvement in Trump’s heavy-handed response, which included a D.C. Guard helicopter flying low over protesters as a show of force.

While then-Defense Secretary Mark T. Esper pushed back behind the scenes against Trump’s desire to call in active-duty troops, Esper came under widespread criticism for describing U.S. cities as a “battlespace” in a White House call with governors. Milley issued an unusual public apology for appearing alongside the president after personnel forcibly cleared protesters from Lafayette Square outside the White House.

The repercussions of those events increased Pentagon leaders’ skepticism of Trump, who since his first day in office has bucked norms for presidential interactions with the military.

“The lesson they took away was: ‘We got caught in the middle of a political firestorm and how do we keep ourselves out of that? The best thing to do is be on the low down, keep a low profile, let’s not get in the mix and let the civilians handle it,’ ” said Risa Brooks, a professor of political science at Marquette University, who studies the U.S. military.

The Pentagon’s impulse to shy away from missions injecting the military into a charged partisan debate backfired in the case of the Capitol riot, Brooks said, because the absence of the military became a political statement in itself for many Americans.

“All they see is: where is the Guard? The Guard was out there with the Park forces out in Lafayette Square ready to come after us, and we weren’t out trying to breach the Capitol . . . that’s what they see,” Brooks said. “And one understands why they see it that way.”

In memos issued ahead of Wednesday’s events, which defense officials provided to The Post, D.C. leaders outlined their request for limited assistance from the D.C. Guard, including traffic control, and specified that military personnel should remain unarmed and refrain from surveilling or arresting protesters.

On Jan. 4, Miller issued a memo to McCarthy placing limits on the D.C. Guard during that mission. The following day, McCarthy issued a memo reiterating the limits to Maj. Gen. William J. Walker, the commanding general of the D.C. Guard.

The directives specifically prohibited the D.C. Guard from receiving ammunition or riot gear for the current mission, using helicopters or surveillance assets, and engaging in searches, seizures, arrests or other law enforcement activity. The directives said the quick reaction force should be deployed only as a measure of last resort.

Mike DeBonis and Dan Lamothe contributed to this report."