The death toll for Friday’s mass shooting and arson at a Russian concert venue near Moscow has risen to at least 133, making it one of the country’s worst terrorist attacks in decades, The New York Times reports.

Russian authorities said they have arrested the four people who committed the attack on Crocus City Hall in Krasnogorsk, where the ’70s rock band Picnic was performing. The Islamic State claimed responsibility for the attack, and U.S. officials believe it can be attributed to the Islamic State in Khorasan (ISIS-K) branch, which is active in Afghanistan and Pakistan.

In footage of the attack shared on social media, people could be heard screaming inside the venue as gunshots rang out. Some videos showed wounded individuals on the ground as weapons were discharged. Others showed the building engulfed in flames, as the gunmen reportedly “threw a grenade or an incendiary bomb,” per Russian news agency RIA Novosti (via CNN).

Picnic and Putin Respond to the Attack

Picnic manager Yuri Chernyshevsky confirmed on Friday that the band had not been harmed. A source from a law enforcement agency told Russian news outlet TASS that the band members had been evacuated from the premises.

The band addressed the brutality in a statement on its website. “The entire Picnic team expresses deep condolences to the families and friends of the victims,” they wrote. “We pray for the speedy recovery of the victims. We are deeply shocked by this terrible tragedy and mourn with you.”

Russian President Vladimir Putin announced that Sunday would be a national day of mourning. Despite the Islamic State claiming responsibility for the attack, Putin accused Ukrainian authorities of preparing a border crossing for the four gunmen and vowed that they would answer for their crimes. “All perpetrators, organizers and commissioners of this crime will receive a just and inevitable punishment,” he said. “No matter who they are, no matter who directed them, I repeat, we will identify and punish everyone who stood behind the terrorists.”