ISIS has claimed responsibility for the attack.
Their statement reportedly read: "A security source told Amaq Agency: Islamic State fighters attacked a large gathering of Christians in the city of Krasnogorsk on the outskirts of the Russian capital, Moscow, killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely."
The gunmen, still reportedly at large, fired at civilians with automatic weapons. Russian news agencies said 50 ambulance crews had been sent to the scene.
Ria news agency in Russia quoted a Moscow official who reported that children were injured when they were performing in a ballroom competition in the same building.
“Children and teenagers were at the competition. People came and said there was shooting. A crush ensued. The children were running out in their ballroom gowns,” they said.
Russia's Federal Security Service, the FSB, released a full statement:
"All possible measures are being taken to provide assistance to those affected as a result of the terrorist attack in the Crocus City Hall.
"The special services are carrying out active search measures following the emergency in the Crocus complex.
"According to preliminary information: 40 dead and over 100 wounded."
Footage showed the building on fire as well. The gunmen reportedly detonated explosives.
An aide to Ukrainian President Volodymyr Zelensky, Mykhailo Podolyak, said in a Telegram post: "let's be clear, Ukraine absolutely has nothing to do with these events."
An eyewitness told the Mash Telegram channel that there were “at least five” attackers and that they were “bearded”.“They act like trained and trained fighters,” the account read. “At the moment of entering the building, the guards and people standing at the door were killed. Then they blocked the main entrance.”
“The terrorists are armed with AKM assault rifles,” the person said. “Some carry unloads with various ammunition. At least two of the attackers are carrying backpacks, possibly with molotov cocktails."
Multiple videos have circulated online of the attack, presumably taken by bystanders. One showed gunmen opening fire in the Crocus City Hall lobby as civilians screamed, ducked and laid on the ground.“The attackers presumably opened fire at the entrance to the building during a concert, using automatic weapons, and then a fire began in the building,” said emergency services.
Another video was taken inside the auditorium, showing the gunman opening fire at people hiding in the seats.
Russian news agency 112 has reported as high as 40 people dead and more than 100 injured. Emergency workers said they had evacuated 100 people who had been sheltering in the building’s basement.
"A terrible tragedy occurred in the shopping center Crocus City today," Moscow Mayor Sergei Sobyanin said. "I am sorry for the loved ones of the victims."
Just over 2 weeks ago, the US embassy in Moscow issued a security alert after receiving reports that "extremists have imminent plans to target large gatherings in Moscow, to include concerts."
At the time, US citizens were advised "to avoid large gatherings over the next 48 hours."
Jack Posobiec, host of Human Events Daily, said on Friday during his show that the attack was reminiscent of the 2015 Paris Bataclan attack.
"We're hearing that Russian response forces are on their way in to try to get people out, try to eliminate whatever threats are currently in there," he said. "This is clearly a Bataclan-style attack, like 2015 in Paris. To my eyes, this is probably the worst Russian terror attack that they've experienced since the Beslan school massacre all the way back in 2004."
This is a breaking story. Refresh for updates.
