Russia’s Federal Security Service (FSB) reported on Saturday that security services detained 11 people in connection with the ISIS-claimed attack on the Crocus City Hall music venue on the outskirts of Moscow that killed at least 133.

The FSB added that four who directly participated in the attack were among the detained. The security service also alleged that the attackers attempted to flee towards Ukraine but were apprehended in the Bryansk region. Ukraine has disavowed any involvement in the attack, which ISIS took responsibility for.

The attackers entered Crocus City Hall just before a concert and opened fire with automatic rifles before setting the building ablaze. Video on social media showed the attackers walking through the building, shooting concertgoers huddled next to the walls. The ensuing conflagration took hours to extinguish amid concerns that the building would collapse, and Russia’s Investigative Committee reported that some of the victims died as a result of smoke inhalation.

US national security spokesperson Adrienne Watson stated on X (formerly Twitter) that the US previously warned Russia about a “planned terrorist attack in Moscow – potentially targeting large gatherings, to include concerts.” The US embassy in Moscow issued an advisory to its citizens on March 7, advising them to avoid large gatherings for 48 hours due to the threat of a terrorist attack.  

In a televised address to the nation, Russian President Vladimir Putin said:

All perpetrators, organizers and clients of this crime will suffer just and inevitable punishment. Whoever they are, whoever guides them. I repeat: we will identify and punish everyone who stands behind the terrorists, who prepared this atrocity, this attack on Russia, on our people.

Russian security services are continuing to investigate.