Here’s what we know so far:
Crocus City Hall
The Russian capital is a sprawling city that is home to some 21 million people.
The Crocus City Hall, which includes a shopping centre and conference venue, lies in suburban Krasnogorsk, about 20km (12 miles) west of the Kremlin and alongside the Moscow ring road.
Opened in 2009, the concert hall is a popular entertainment venue with a capacity for 6,200 people.
Former United States President Donald Trump once held a Miss Universe contest there.
The attack
The attack began on Friday evening just as people were taking their seats for a sold-out show by Picnic, a popular rock band from the Soviet era.
As many as five men in combat fatigues entered the concert hall and opened fire on those inside.
Dave Primov, who was in the hall during the attack, described chaotic scenes.
“There were volleys of gunfire,” Primov told The Associated Press news agency. “We all got up and tried to move toward the aisles. People began to panic, started to run and collided with each other. Some fell down and others trampled on them.”
Russian investigators said more than 60 people had been killed, while 145 more were injured in one of the worst attacks to hit the country in decades.
The attackers also set off explosives that ignited a huge blaze that at one point covered as much as 12,900 square metres (139,000sq ft), according to Russian news agency Interfax.
Helicopters were brought in to douse the flames from the air, as firefighters battled the blaze from the ground. The fire was eventually brought under control early on Saturday.
The Emergency Situations Ministry said firefighters helped about 100 people escape through the building’s basement, while rescue operations were also launched for people trapped on the roof.
TASS news agency said the members of Picnic were not harmed and were evacuated safely.
The hunt for the attackers
Russia’s Investigative Committee, the top state criminal investigation agency, opened a “terrorist” investigation into the attack and the national guard, Rosgvardia, was among units deployed to search for the gunmen.
Graphic videos posted on social media showed the gunmen firing repeated rounds as they entered the building, shooting screaming people at point-blank range.
Another video showed a man in the auditorium saying the attackers had set it on fire.
The Kremlin did not immediately blame anyone for the attack, but some Russian lawmakers were quick to accuse Ukraine.
Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, denied Ukraine’s involvement.
“Ukraine has never resorted to the use of terrorist methods,” he posted on X. “Everything in this war will be decided only on the battlefield.”
ISIL claims responsibility
Islamic State in Khorasan Province (ISKP), an Afghan affiliate of ISIL (ISIS), claimed responsibility for the attack in a statement posted by its Amaq news agency.
It said its fighters had attacked on the outskirts of Moscow, “killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely” and then escaped. It gave no further detail.
Russia has reported several incidents involving ISIL this month, with the FSB intelligence agency saying on March 7 it foiled an attack by ISKP on a Moscow synagogue.
The US had also warned of the heightened threat posed by “extremists” with imminent plans for an attack on “large gatherings” in Moscow, and had shared that finding with the Russians. On Friday night, a US official said Washington had intelligence confirming ISIL’s claim of responsibility for the Crocus City Hall attack.
Experts said the group had opposed Russian President Vladimir Putin in recent years.
“ISIS-K [ISKP] has been fixated on Russia for the past two years, frequently criticising Putin in its propaganda,” said Colin Clarke of Soufan Center, a Washington-based research group.
Michael Kugelman of the Washington, DC-based Wilson Center said that ISKP “sees Russia as being complicit in activities that regularly oppress Muslims”.
What previous attacks have targeted Russia?
In October 2015, an ISIL-planted bomb exploded on a Russian passenger plane over Sinai, killing all 224 people on board, most of them Russians returning from their holidays in Egypt.
The country was also shaken by a series of deadly attacks in the early 2000s.
In September 2004, about 30 Chechen fighters seized a school in Beslan in southern Russia taking hundreds of people hostage. The siege ended in a bloodbath two days later and more than 330 people, about half of them children, were killed.
In October 2002, Chechen fighters stormed a theatre in Moscow taking about 800 people hostage in the auditorium. Russian special forces launched a bid to rescue the captives two days later, by first subduing the attackers with a narcotic gas. Some 41 Chechens died, as well as 129 hostages, mostly from the effects of the gas.