What We Know About the Moscow Concert Hall Attack

An attack Friday at a popular concert venue near Moscow killed 137 people, the deadliest act of terrorism the Russian capital region has seen in more than a decade.

The Islamic State claimed responsibility for the attack; American officials have attributed it to ISIS-K, a branch of the group.

Russian officials and state media have largely ignored ISIS’s claim of responsibility and instead suggested that Ukraine was behind the violence. Ukraine has denied any involvement, and American officials say there is no evidence connecting Kyiv to the attack.

Russian authorities have detained at least 11 people, including four migrant laborers in Russia who have been charged with committing a terrorist act, but they have not identified most of the accused assailants or their motives.

Here’s a closer look at the attack.

The gunmen entered the Crocus City Hall building, one of the biggest entertainment complexes in the Moscow area, with capacity of more than 6,000, shortly before a sold-out rock concert was scheduled to start. Armed with automatic rifles, they began shooting.

Using explosives and flammable liquids, Russian investigators said, they set the building ablaze, causing chaos as people began to run. The fire quickly engulfed more than a third of the building, spreading smoke and causing parts of the roof to collapse. Russia’s emergency service posted a video and pictures from after the fire showing charred seating and firefighters working to remove debris.

Russian law enforcement said that people had died from gunshot wounds and poisoning from the smoke.

At least three helicopters were dispatched to extinguish the fire or to try to rescue people from the roof. The firefighters were only able to contain the fire early on Saturday; the emergency service said it was mostly extinguished by 5 a.m.

The search for survivors ended on Saturday, as details about the victims began to emerge. Many of the more than 100 people injured in the attack were in critical condition.

Russia’s Investigative Committee, a top law-enforcement body, said on Sunday that 137 bodies had been recovered from the charred premises, including those of three children. It said that 62 victims had been identified so far and that genetic testing was underway to identify the rest.

Attackers were able to flee the scene. Early on Saturday, the head of Russia’s top security agency, the F.S.B., said that 11 people had been detained in the connection to the attack, including “all four terrorists directly involved.” The four men were arraigned late Sunday and charged with committing a terrorist act, according to state and independent media outlets, and they face a maximum sentence of life in prison.

The press service of the Basmanny District Court said that the first two defendants, Dalerjon B. Mirzoyev and Saidakrami M. Rachalbalizoda, had pleaded guilty to the charges.

It did not specify any plea from the other two — Muhammadsobir Z. Fayzov, a 19-year-old barber and the youngest of the men charged, and Shamsidin Fariduni, 25, a married factory worker with an 8-month-old baby — according to Mediazona, an independent news outlet.

The men looked severely battered and injured as they appeared in court, and videos of them being tortured and beaten while under interrogation circulated widely on Russian social media.

There were signs that Russia would try to pin blame on Ukraine, despite the claim of responsibility by the Islamic State. The F.S.B. said in a statement that the attack had been carefully planned and that the terrorists had tried to flee toward Ukraine.

President Vladimir V. Putin, who claimed victory in a presidential election last weekend, did not publicly address the tragedy until Saturday afternoon. In a five-minute address to the nation, he appeared to be laying the groundwork to blame Ukraine for the attack, claiming that “the Ukrainian side” had “prepared a window” for the attackers to cross the border from Russia into Ukraine.

But he did not definitively assign blame, saying that those responsible would be punished, “whoever they may be, whoever may have sent them.”

The attack has punctured the sense of relative safety for Muscovites over the past decade, bringing back memories of attacks that shadowed life in the Russian capital in the 2000s.

Russia observed a national day of mourning on Sunday as questions lingered about the identities and motives of the perpetrators. Flags were lowered to half-staff at buildings across the country.

Neil MacFarquhar contributed reporting.