WASHINGTON — A state lawmaker from West Virginia, a 70-year-old Alabamian armed with jars of gasoline and a man who broke into Speaker Nancy Pelosi’s office and posed at a desk there were among those arrested after the storming of the Capitol, federal law enforcement officials announced on Friday as they promised an exhaustive investigation into the violence.
The sweeping investigation took shape after a security breakdown two days earlier allowed hundreds of people backing President Trump’s efforts to overturn the election results to rush into the Capitol in a deadly rampage. The debacle prompted sharp recriminations and the firings of top officials who were entrusted with protecting lawmakers. Dozens have been arrested, including 13 who face federal charges.
Law enforcement officials also backed off a suggestion that Mr. Trump could face criminal charges for inciting the riot after a top prosecutor had said a day earlier that investigators were examining anyone involved, “not only” the rioters.
“Don’t expect any charges of that nature,” Ken Kohl, a top prosecutor in the U.S. attorney’s office in Washington, told reporters in a telephone briefing on Friday.
Law enforcement officials also sought to explain the security failure, saying that they had no indication that the day would turn violent. But Trump supporters had for weeks openly discussed on social media their plans to protest Congress’s certification of the Electoral College results, a typically ceremonial affair, and in some cases pledged to fight for their cause.
The number of arrests is likely to grow quickly as investigators pore over social media to identify the rioters. And some could face more serious charges, including in the death of Brian D. Sicknick, a Capitol Police officer who was overpowered by rioters who, according to two law enforcement officials, struck his head with a fire extinguisher. He was rushed to the hospital and died on Thursday.
Hundreds of prosecutors and F.B.I. agents have been assigned to work the investigation and were pursuing dozens of cases, Mr. Kohl said.
“We are far from done,” added Steven M. D’Antuono, who runs the F.B.I.’s Washington field office.
He also rebutted the notion of any involvement in the violence by left-wing antifascist agitators, whom Mr. Trump’s supporters have falsely tried to blame.
The investigation is likely to span weeks and stretch across the country as the F.B.I. seeks to identify those who were involved in the melee. U.S. attorneys and the F.B.I. were pursuing conspiracy charges against people who might have traveled together and then broken into the Capitol, and some already charged were from Florida, Maryland, Illinois, Virginia, Pennsylvania and elsewhere.
“The goal here is to identify people and get them,” Mr. Kohl said.
The Washington police have also arrested dozens, mostly on charges of unlawful entry and curfew violations.
One of the most serious federal cases involved Lonnie L. Coffman of Falkville, Ala. In the bed of his truck, officers found what they described as an M4 assault rifle and magazines loaded with ammunition. They also found rags, lighters and 11 glass Mason jars filled with a liquid later identified as gasoline.
Bomb technicians determined that they were meant to be turned into Molotov cocktails — small, hand-thrown fire bombs. Mr. Coffman was arrested when he tried to return to his truck around 6:30 p.m. Wednesday. The police found two handguns in his pants pockets, a 9-millimeter handgun in his right front pocket and a .22-caliber pistol in his left front pocket.
When questioned by the police, Mr. Coffman said his Mason jars contained “melted Styrofoam and gasoline,” which would have behaved similarly to napalm when ignited.
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Also among those arrested was Derrick Evans, a newly elected lawmaker from West Virginia, Mr. Kohl said. Mr. Evans posted video to his Facebook page of him filming as he stood among the crowd outside a Capitol door, shouting, “There we go! Open the door!” and chanting, “Our house! Our house!” before rushing inside. “We’re going in!” he added.
Though he deleted the video, the F.B.I. found a copy on Reddit.
Another man taken into custody, Richard Barnett, 60, from Gravette, Ark., had posted a picture on social media of himself sitting at a desk in Ms. Pelosi’s office with his feet up and had said he expected to be arrested. “I’ll probably be telling them this is what happened all the way to the D.C. jail,” Mr. Barnett told a reporter later on Wednesday.
The images of Mr. Barnett were “shocking” and “repulsive,” said Jeffrey A. Rosen, the acting attorney general.
Mr. Barnett faces three criminal counts that included knowingly entering or remaining in any restricted building or grounds without lawful entry; violent entry and disorderly conduct on Capitol grounds; and theft of public money, property or records. He was expected to make an initial appearance in Little Rock, Ark., and then be sent to Washington to face prosecution.
The arrests clearly implicated Trump supporters. Prosecutors charged the leader and founder of the Hawaii chapter of the far-right Proud Boys group, Nicholas Robert Ochs, with unlawful entry after he posted a picture on Twitter from the Capitol and told a CNN reporter that he had gone inside.
The Proud Boys, whose name is tattooed on Mr. Ochs’s arm, have vocally supported Mr. Trump’s efforts to overturn the election results. “Some members have advocated for violent action to achieve these ends,” an F.B.I. agent said in court documents.
The federal authorities were still working to identify and arrest the person who placed a pair of explosive devices near the Capitol and offered a $50,000 reward for information leading to the arrest of a person seen in a hoodie and pulling a bag.
A pair of pipe bombs found on Wednesday afternoon outside the Democratic and Republican Party headquarters, blocks from the Capitol, contained crude mechanical timing devices, according to an official familiar with their initial examination, suggesting they were intended to be detonated. It was not clear when they were meant to explode.
The two bombs appeared to be similar, the official said. They were relatively unsophisticated and incorporated a mechanical timer, steel wool and an unknown powder as their explosive fill, the official said. One of the devices was found with a cellphone, which could have been incorporated to detonate the bomb.
Bomb squad technicians disabled the devices, which were constructed of steel pipes that appear to be about one and a half inches in diameter and 12 inches long.
Mr. Coffman is not considered a suspect in the placement of those bombs, according to an affidavit.
As for the president, Justice Department officials reversed course one day after refusing to rule out the possibility of pursuing charges against him. On Friday, Mr. Kohl said that no charges would be filed against anyone who made inflammatory comments at the rally on Wednesday near the White House where the president exhorted his supporters to march to the Capitol.
A Justice Department spokesman later reiterated that prosecutors had not charged anyone with insurrection or incitement and that their focus “is on the events at the Capitol.”
Other close allies of the president made similar comments at the rally. His eldest son, Donald Trump Jr., said that Republicans should back Mr. Trump’s efforts to undo the election result or face consequences. “We’re coming for you,” he said.
And Rudolph W. Giuliani, Mr. Trump’s personal lawyer, told the president’s supporters that they would need to engage in “trial by combat” against Democrats if they wanted to overturn the election results.
Incitement of a riot is a misdemeanor crime in Washington that carries up to 180 days in prison or a $1,000 fine, but the maximum sentence increases to 10 years if victims suffer serious bodily harm or serious property damage occurs.
The notion that Mr. Trump incited the attack on Congress underpins articles of impeachment that Democrats in the House are drawing up. They have threatened to impeach Mr. Trump a second time if Vice President Mike Pence refuses to move to strip Mr. Trump of his powers under the 25th Amendment.
Katie Benner contributed reporting.