Russia Observes Day of Mourning for Victims

American officials attribute Friday’s attack in a Moscow suburb, which killed at least 137 people, to a branch of the Islamic State active in Iran and Afghanistan.

Here’s what to know about the attack and its aftermath.

Russia was observing a national day of mourning on Sunday for victims of a fiery terrorist attack on a suburban Moscow concert venue, as the death toll climbed to 137 and questions lingered about the identities and motives of the perpetrators.

Under a gray sky, people came to lay flowers and light candles at a memorial set up outside the concert hall. Scores of people waited in a long line for their turn, many clutching red bouquets, as efforts were underway inside to dismantle the remains of the stage. Flags were lowered to half-staff at buildings across the country, and state media released a video of President Vladimir V. Putin lighting a memorial candle in a church.

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.

There are two primary narratives about the violence on Friday night, Russia’s deadliest terrorist attack in 20 years.

American officials say it was the work of Islamic State Khorasan, or ISIS-K, an Islamic State offshoot that has been active in Pakistan, Afghanistan and Iran.

But on Saturday, Mr. Putin did not mention ISIS-K or the identities of the perpetrators in his first public remarks on the tragedy. Russian state news broadcasts have largely ignored or cast doubt on the ISIS theory, instead laying the groundwork to suggest that Ukraine and its Western backers were responsible. Ukraine has denied any involvement.

Here’s what to know:

• The search for survivors ended on Saturday, as details about the victims began to emerge. Many of the more than 100 people wounded in the attack were in critical condition.
• Russia has not said much about four people it has arrested and considers the main suspects in the attack. The Interior Ministry has said only that they are foreign citizens. The state news media has shown what it described as footage of an interrogation of a suspect who spoke in Tajik through an interpreter.
• As Russia mourned, the war in Ukraine carried on. Ukraine’s air force said it had shot down 43 out of 57 Russian missiles and drones launched overnight against different parts of the country. And Ukraine’s military said it had struck two large landing ships that were part of Russia’s Black Sea Fleet. There was no immediate comment from Russia’s defense ministry.
• Piknik, the Russian rock band that was to play a sold-out concert at the suburban venue on the night it was attacked and burned to rubble, now finds itself at the center of the tragedy.
• The attack dealt a political blow to Mr. Putin, a leader for whom national security is paramount.