Cutting through the cross talk, one person on the call posed a blunt question: “Chief Sund, are you requesting National Guard troops on the grounds of the Capitol?”

There was a pause.

“Yes,” Chief Sund replied, “I am.”

Yet the Capitol Police and the city’s Metropolitan Police had rebuffed offers days before for more help from the National Guard beyond a relatively modest contingent to provide traffic control, so no additional troops had been placed on standby. It took just over four hours for them to arrive.

It was just one failure in a dizzying list that day — and during the weeks leading up to it — that resulted in the first occupation of the United States Capitol since British troops set the building ablaze during the War of 1812. But the death and destruction this time was caused by Americans, rallying behind the inflammatory language of an American president, who refused to accept the will of more than 81 million other Americans who had voted him out of office.

A mob surrounded the Capitol on Wednesday after a Trump rally nearby. Jason Andrew for The New York Times

President Trump’s call at a rally that day for the crowd to march on the nearby Capitol was surely a spark that helped ignite the deadly riots that left five dead — including a policeman and a woman who stormed the building — injured dozens of others and damaged the country’s reputation for carrying out peaceful transfers of power. But the tinder for the blaze had been gathering for months, with every tweet that the election had been stolen, every refusal by Republican lawmakers to recognize Joseph R. Biden Jr. as the next president, every dog-whistle call that emboldened white supremacist groups to violently strike.

A full reckoning will take months or even years, and many lawmakers have called for a formal commission to investigate.

But an initial anatomy of the siege by The New York Times revealed numerous failures. The chaos showed that government agencies have no coordinated plan to defend against an attack on the Capitol — especially one specifically aimed at powerful elected officials — though law enforcement agencies have for years raised alarms about the growing threat of domestic terrorism. QAnon, an online conspiracy group that was well represented among the crowd, has been labeled a domestic terrorist threat by the F.B.I.

Speaker Nancy Pelosi and Vice President Mike Pence presided over the joint session of Congress that was disrupted by rioters storming the Capitol. Pool photo by Erin Schaff

Federal agencies and the Capitol Police appeared to issue no serious warnings in the days leading up to the riots that the gathering could turn violent, despite countless posts on right-wing social media sites pledging confrontation and even bloodshed.

The Department of Homeland Security invited local law enforcement agencies to its situation room — held online during the pandemic — only the day before the riots, which some security experts said was far too late.

Poor planning and communication among a constellation of federal, state and local law enforcement agencies hamstrung the response to the rioting. Once the Capitol building was breached, a patchwork group of reinforcements was forced to try to navigate a labyrinthine complex of unfamiliar passages and byways that would prove dangerous.

Above all, the fiasco demonstrated that government agencies were not prepared for a threat that, until recently, seemed unimaginable: when the person inciting the violence is the president of the United States.

The Capitol Police and the Metropolitan Police Department did not respond to requests for comment. Ms. Bowser’s chief of staff, John Falcicchio, said that defense officials determined the number of personnel deployed. But Pentagon officials said they made those decisions based on the specific requests they received.

The recriminations began almost immediately, and the violence also carried a sobering reality: The country got lucky. Hundreds of rioters carrying weapons breached the seat of American power — some with the clear intent of injuring, holding hostage or even killing federal officials to stop them from certifying the vote. In the end, all of the lawmakers were spirited away to safety.

“It was such an embarrassingly bad failure and immediately became an infamous moment in American history,” said R.P. Eddy, a former American counterterrorism official and diplomat who now runs a private intelligence firm. “But it could have been so much worse.”

Missed Warnings

“The Capitol is our goal. Everything else is a distraction,” announced one post on far-right social media a day before the uprising. “Every corrupt member of Congress locked in one room and surrounded by real Americans is an opportunity that will never present itself again.”

That was just one example of how extremists were organizing on social media.

In private Facebook groups, activists planning to make the trip to the capital discussed not only logistics like hotels and rideshares, but also sleeping in cars and pitching tents should they need to “occupy” the city. Many comments included photographs of guns and ammunition that they planned to bring.

Trump supporters broke windows to breach the building. Erin Schaff/The New York Times

On smaller social media platforms such as Parler and Gab that became rallying places for the far right, calls for violence were more overt. Dozens of posts in the days leading up to Wednesday listed assault rifles and other weapons that people claimed they were bringing to Washington. People discussed which types of ammunition were best and whether medics would be in place for those potentially injured.

Law enforcement and other officials were aware of the chatter and took some steps to try to reduce the chances of violence. Homeland security officials put tactical agents on standby in downtown Washington. The F.B.I. questioned neo-Nazis who were under investigation and planning to attend the demonstrations, prompting some of them to change their plans and skip the trip, officials said.

And on Monday, the Metropolitan Police Department arrested Enrique Tarrio, the leader of the Proud Boys, a far-right group. The police also announced before the rally that anyone who planned to show up to the demonstrations armed, in violation of local firearms laws, would be arrested.

But missed opportunities abounded. Despite the ominous social media posts, officials leading intelligence-sharing centers throughout the United States received no warnings from the federal government about the potential threat to the Capitol.

“We did not see any federal products related to this,” said Mike Sena, the president of the National Fusion Center Association. Such centers were formed after the Sept. 11, 2001, attacks to improve communication and planning among federal and local agencies.

One senior federal prosecutor in the Midwest said he did not even speak with the top F.B.I. agent in his city about local residents possibly traveling to Washington. What the F.B.I. had been observing online, officials would later say, amounted to First Amendment-protected activity, despite the incendiary language in social media posts.

Intelligence experts denounced the inability — or refusal — of government analysts to provide proper warning about impending violence.

“The evidence is starkly clear that the momentum of violence has shifted to the right in this country. We’ve seen this in city after city,” Mr. Eddy said. “There was a failure among law enforcement to imagine that people who ‘look like me’ would do this.”

Chase Jennings, a spokesman for the Homeland Security Department, said in the days leading up to the breach at the Capitol, the agency “had open channels with partners and shared information on those channels.”

Rioters who stormed the Capitol confronted law enforcement officials inside. Erin Schaff/The New York Times

Protests Last Year

Looming over preparation was the government’s heavy-handed response to the Black Lives Matter demonstrations over the summer. Mr. Trump had deployed unidentified agents and tactical teams to the streets of Washington in June against the wishes of Ms. Bowser, and agents tear-gassed protesters, allowing the president to walk to a nearby church and stage a photo op holding a Bible. In the run-up to the violence on Wednesday, the fractured relationship between federal and local law enforcement was evident.

Ms. Bowser had sent a letter to top federal law enforcement officials on Tuesday warning against excessive deployments. Though the city had sought some National Guard troops for traffic control, she noted that the D.C. police had not requested additional personnel from law enforcement agencies for the rallies on Wednesday and referred to the aggressive deployment in June.

Still, federal law enforcement officials conveyed to lawmakers that they were prepared. David L. Bowdich, the F.B.I. deputy director, assured Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, that the bureau had the resources to handle the Trump rally.

The Justice Department was treating the event as relatively peaceful, officials said. The acting attorney general, Jeffrey A. Rosen, worked on Wednesday from his office rather than the F.B.I. war room, where the crisis response unfolded.

Several officials, including Chief Sund of the Capitol Police, briefed Representative Zoe Lofgren, Democrat of California and the chairwoman of the House Administration Committee, on Tuesday. The message was similar to the one that Chief Sund had given to other lawmakers for days.

“I was told by the police chief and the sergeant-at-arms that everything is under control and they had provided for every contingency,” Ms. Lofgren said in an interview. “That turned out to be completely false.”

‘An Active, Hostile Situation’

Within minutes of the mob breaching the Capitol complex, rioters were pounding on the doors of the House gallery, where a group of nearly two dozen lawmakers were trapped. The sounds of shattering glass echoed through the chamber.

“I thought we’d either have to make a last stand or fight our way out,” said Representative Jason Crow, Democrat of Colorado and a former Army Ranger who served in Iraq. “It was pretty hairy.”

Representative Jason Crow, Democrat of Colorado, took shelter in the House gallery as the mob tried to break into the chamber. Andrew Harnik/Associated Press

An outnumbered force of Capitol Police had tried numerous tactics to keep the riot at bay, setting up barricades, using pepper spray and trying to push back against the mob at the building’s doors and windows. All of these measures failed.

Lawmakers had no clear evacuation plan and were forced to improvise.

Mr. Crow said he moved other lawmakers away from the barricaded door inside the gallery, helped them don hoods to protect against tear gas, had them remove their House lapel pins to avoid being targeted and took out his only possible weapon: a pen.

After nearly 30 minutes, he said, the Capitol Police and unidentified SWAT team officers cleared a path outside the gallery, above the House floor, and hustled out the lawmakers.

With the police in the lead, guns drawn, the group stumbled through the mayhem, Mr. Crow said. Some police officers rushed to barricade other doors to block the mob. Others pinned some rioters to the ground to allow the lawmakers to pass.

Because of efforts to restrict the number of people in the chamber, several lawmakers and aides were sheltering in their offices, scattered across the complex. Some were not contacted by the police, even as they barricaded themselves inside.

Many of the House members remained in one secure location, where they might have been exposed to someone with the coronavirus, the Office of the Attending Physician said on Sunday.

Representative Lisa Blunt Rochester, Democrat of Delaware, pushed a handful of Republicans to wear masks, to no avail. Representatives Hakeem Jeffries, Democrat of New York, and Liz Cheney, Republican of Wyoming, periodically provided updates to the room, as lawmakers called their families and checked on their staff members.

On the Senate side of the Capitol, the rioters came perilously close to lawmakers. As they approached, a quick-thinking Capitol Police officer pushed one of them, then backed away, and the crowd chased him. The officer’s maneuver helped lead the mob away from an entrance to the Senate several feet away, according to a video taken by Igor Bobic, a HuffPost reporter.

In a secure, undisclosed location, Senator Lindsey Graham, Republican of South Carolina, screamed at the Senate sergeant-at-arms, Michael C. Stenger, demanding a plan and ordering him to clear the rioters, according to a person in the room. Mr. Stenger was milling around, the person said, inspiring no confidence that he was in control of the situation. He has since resigned, as has Chief Sund. Throughout the Capitol, urgent voices crackled across police radios giving details about the unfolding siege.

“There was definitely a higher sense of urgency” on police radio traffic as rioters breached the east side of the Capitol, said Ashan M. Benedict, the head of the Washington field office of the Bureau of Alcohol, Tobacco, Firearms and Explosives, who was working with the Capitol Police at the nearby Republican Party headquarters, where a pipe bomb was found.

Mr. Benedict connected with a commander of the Capitol Police SWAT team who was inside the complex, who acknowledged that they needed immediate help but said he needed a moment to arrange the official request.

The Capitol Police detained rioters outside the House chamber. Drew Angerer/Getty Images

A.T.F. and F.B.I. teams were soon headed to the Capitol. Neither bureau trains its agents for crowd control or riots, and they would have to find a way in, where they could help clear the Capitol and rescue staff members and employees.

When Mr. Benedict and his deputy finally got into the building, it was madness, he recalled. Clouds of noxious gas — bear spray, he guessed, from rioters — floated through the halls. With the help of a Capitol Police officer, they helped usher their teams through a growing crowd of rioters on the building’s south side.

“We walked into an active, hostile situation,” Mr. Benedict said. “Some were thanking us for our service. And some were asking us not to go in and protect the Capitol, and stand with them.”

Chaos and Confusion

Thirty-three miles away, in Annapolis, Md., Gov. Larry Hogan, a Republican, was on a video conference call with the Japanese ambassador when his chief of staff rushed into his office, telling him, “The Capitol is under attack.”

The governor’s phone rang minutes later: a call from Representative Steny H. Hoyer of Maryland, the House majority leader.

“He’s fired up. He’s agitated. He’s stressed,” Mr. Hogan recalled in an interview. “He says, ‘Governor, I’m in a room with Nancy Pelosi and Chuck Schumer. We need help. What can you do?’”

By then, the mob had breached the security outside the building, and Ms. Bowser and her staff members had begun making urgent calls to mobilize larger numbers of D.C. National Guard troops and move those already in the city to the Capitol. In the days before, Ms. Bowser had requested only a relatively small contingent of 340 D.C. National Guard troops, and only to control traffic and help protect public transportation stops, an effort to avoid the militarized federal presence that had been a major factor in the protest response in June.

One of the calls was to Mr. Hogan, asking the governor to dispatch Maryland National Guard troops to the city. Because Washington, D.C., is not a state, the new request needed to be approved by the acting secretary of defense, Christopher C. Miller.

Mr. Hogan’s phone rang. It was Ryan D. McCarthy, the secretary of the Army and the de facto head of the D.C. National Guard. He asked whether Maryland troops could come immediately.

“I said, ‘Yes, we’ve been waiting,’” Mr. Hogan recalled.

Ms. Bowser was having similar problems. Even during the phone call when Chief Sund said he needed National Guard troops to beat back the rioters — a request the mayor and her staff members figured would immediately prompt an order of reinforcements — Pentagon officials would not commit to sending them.

Rioters climbing the Capitol walls. Jason Andrew for The New York Times

Lt. Gen. Walter E. Piatt, the director of the Army staff, who was on that call, reacted to Chief Sund’s request with caution. He said that he did not have the authority to send the troops, that the request would have to go through his chain of command, and that the group needed a plan for how the National Guard would be deployed.

Chief Robert J. Contee of the Metropolitan Police was livid.

“Are you denying the request?” he asked General Piatt three times.

“We are not denying the request,” the general insisted. But, he added, he would have to seek approval first. The phone call ended.

Inside the mayor’s command center, where officials recalled the debacle in June when the military sent a helicopter to Black Lives Matter protests, frustration turned to anger.

“The Capitol Police were requesting the guard, they were not getting the request fulfilled, and we are seeing blood on the ground of the United States Capitol. That was the moment for me,” said Mr. Falcicchio, the mayor’s chief of staff.

In an interview, General Piatt defended his caution.

“The last thing you want to do is throw forces at it where you have no idea where they’re going, and all of a sudden it gets a lot worse,” he said.

Inside the besieged Capitol, lawmakers were making their own urgent requests to the Pentagon. Representative Elissa Slotkin, Democrat of Michigan and a former defense official, called Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, to plead for help from the National Guard.

“What I was hearing is that the Pentagon was really struggling with the request,” she said, because of the response to the protests in June, which had brought intense criticism upon the Defense Department and General Milley.

“We all should acknowledge that this was a tough decision” because of the violence last year, Ms. Slotkin said. “I said, ‘Mark, I absolutely understand that you are between a rock and a hard place. But we need help here.’”

Finally, at about 3 p.m., Mr. Miller decided that all available D.C. National Guard soldiers — 1,100 troops — would be deployed.

Help Arrives

As the Pentagon deliberated, city officials sent an urgent dispatch to all local police forces asking for help.

Ed Roessler, the police chief in Fairfax County, Va., was driving when his phone rang at 2:27 p.m. A deputy told him that a request for help had just gone out over the police mutual aid radio system used by law enforcement agencies in the Washington region. He was stunned.

“It was surreal,” said Chief Roessler, a 32-year veteran of the department. “To get a request over the police mutual aid radio system — I don’t recall something at this level.”

About 40 Fairfax County officers were quickly deputized by an official from the United States marshals and sent to Washington. At the Capitol, they assembled in a wedge formation and went inside, where they helped push out rioters.

Then they stood guard while officers from other agencies built a larger security perimeter around the Capitol.

Other requests for help went out, and the Capitol Police also sought assistance from the Homeland Security Department, but not until more than an hour after the rioters had surrounded the Capitol and the police had first fired what appeared to be flash-bang grenades.

When the request came at 2:30 p.m., the Secret Service deployed both uniformed and special agents, according to Kenneth T. Cuccinelli II, the acting deputy secretary of homeland security, the parent agency of the Secret Service.

Asked if he wondered why the request from the Capitol Police came at 2:30, Mr. Cuccinelli said, “It’s pretty clear they were underprepared, unfortunately.”

Placing Blame

National Guard troops patrolled the Capitol over the weekend. Anna Moneymaker for The New York Times

When the rioters breached the Capitol, Senator Kevin Cramer, Republican of North Dakota, said a quick prayer.

As he and the other senators made their way out of the chamber to the basement, an officer urged them to hurry because the rioters were on their heels.

“‘Move quicker, people,’ the officer said. ‘They’re right behind.’ It was serious,” Mr. Cramer recalled.

Out of immediate danger, senators took roll call. Four were missing, including Senator Tammy Duckworth, Democrat of Illinois, who uses a wheelchair after sustaining injuries in Iraq. She had barricaded herself in her office.

Inside the secure location, some senators grew increasingly angry at Senators Ted Cruz of Texas and Josh Hawley of Missouri — Republicans who had vowed to fight the election certification unless a commission was established to investigate Mr. Trump’s baseless claims of voter fraud.

Senator Joe Manchin III of West Virginia, a moderate Democrat, said he approached Senators Steve Daines of Montana and James Lankford of Oklahoma, Republicans who had planned to object to the election to send a message.

“‘Steve, c’mon, you don’t want to be part of this,’” Mr. Manchin recalled saying. “I said, ‘James, you’re better than this.’”

Mr. Manchin said the appeals affected them, but Mr. Hawley showed no remorse.

“Josh Hawley started the whole thing, and all who assisted him, they’ve got to be held accountable,” Mr. Manchin said.

When asked to respond, spokespeople for Mr. Hawley said he had quickly condemned the violence and never claimed there was widespread election fraud, making only a narrow argument about mail balloting in Pennsylvania. Mr. Hawley has declined interviews in the aftermath of the riot.

For others, the blame goes far wider.

Senator Amy Klobuchar, Democrat of Minnesota, said she and other senators were investigating what went wrong and focusing on how to keep Mr. Biden’s inauguration safe.

“There clearly needs to be an overhaul of security,” she said, adding that ultimately, the blame fell on the president of the United States.

“He convinced them this was a righteous cause, to be part of an insurrection,” Ms. Klobuchar said. “And they went for it.”

Reporting was contributed by Eric Schmitt, Adam Goldman, Emily Cochrane and Katie Benner from Washington, Ali Watkins, Ashley Southall and Shaila Dewan from New York, and Sheera Frenkel from Oakland, Calif.

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Correction: Jan. 10, 2021
An earlier version of the homepage summary of this article incorrectly stated when British troops set the Capitol ablaze. It was in 1814, during the War of 1812, not in 1812.

Mark Mazzetti is a Washington investigative correspondent, a job he assumed after covering national security from the Washington bureau for 10 years. He was part of a team that won a Pulitzer Prize in 2018 for reporting on Donald Trump’s advisers and their connections to Russia.

Helene Cooper is a Pentagon correspondent. She was previously an editor, diplomatic correspondent and White House correspondent, and was part of the team awarded the 2015 Pulitzer Prize for International Reporting, for its coverage of the Ebola epidemic.

Jennifer Steinhauer has been a reporter for The New York Times since 1994. She has worked on the Metro, Business and National desk, and served as City Hall bureau chief and Los Angeles bureau chief before moving to Washington in 2010. She is the author of a novel, two cookbooks and "The Firsts" the story of the women of the 116th Congress.

Zolan Kanno-Youngs is the homeland security correspondent, based in Washington. He covers the Department of Homeland Security, immigration, border issues, transnational crime and the federal government's response to national emergencies and security threats.

Luke Broadwater covers Congress. He was the lead reporter on a series of investigative articles at the Baltimore Sun that won a Pulitzer Prize and a George Polk Award in 2020.

A version of this article appears in print on Jan. 11, 2021, Section A, Page 1 of the New York edition with the headline: Flood of Failures Let Mob Rampage Through Capitol .