MOSCOW, Russia (AFP) — Russian President Vladimir Putin said Saturday that the four gunmen who killed more than 100 at a Moscow concert hall had been arrested, and vowed harsh retribution for everybody involved in plotting the attack.

“All four perpetrators of the terrorist act who shot and killed people have been detained. They were travelling towards Ukraine … We will identify and punish everybody who stood behind the terrorists, who prepared the attack.”

Russia said it had arrested 11 people — including four gunmen — over Friday’s attack on the Crocus City Hall which left at least 115 dead. The attack has been claimed by the Islamic State jihadist group.

Camouflaged gunmen opened fire at the packed hall in Moscow’s northern suburb of Krasnogorsk ahead of a concert by rock band Piknik in the deadliest attack in Russia for at least a decade.

UN Secretary General Antonio Guterres and the UN Security Council made the “strongest” condemnations of the attack and offered condolences to the victims of the “terrorist” incident.

Meanwhile, UN human rights chief Volker Turk said he was “horrified” by the incident and called for the perpetrators to be held to account.