Four men have been charged with acts of terrorism after an attack at a Moscow concert hall killed at least 137 people and left more than 100 injured. Three men were brought into a Moscow court bent over while the fourth was in a wheelchair, according to the BBC.

The attack took place on March 22, when a group of gunmen allegedly opened fire at the Crocus City Hall on the outskirts of Moscow, as per reports from Russian state media according to The New York Times.

It happened ahead of a scheduled performance by the Russian rock band Picnic, and also resulted in a major fire across the building. The Islamic State group claimed it carried out the attack, but Russian officials have claimed Ukrainian involvement – a claim Kyiv said is “absurd.”

Arrests were made following the attack, and the four men were named by Russian authorities as Dalerdzhon Mirzoyev, Saidakrami Murodali Rachabalizoda, Shamsidin Fariduni and Muhammadsobir Fayzov.

Russian president Vladimir Putin vowed to “identify and punish” those responsible. “The main thing now is to prevent those who were behind this bloody massacre from committing new crimes,” he said.