Four men accused of staging the Russia concert hall attack that killed more than 130 people appeared before a Moscow court Sunday on terrorism charges.

The investigators charged Dalerdzhon Mirzoyev, 32; Saidakrami Rachabalizoda, 30; Shamsidin Fariduni, 25; and Mukhammadsobir Faizov, 19, with committing a terrorist attack resulting in the death of others. The offense carries a maximum sentence of life imprisonment.

Moscow’s Basmanny District Court ordered that the men, all of whom were identified in the media as citizens of Tajikistan, be held in custody until May 22 pending investigation and trial.

The hearings came as Russia observed a national day of mourning for the attack Friday on the suburban Crocus City Hall concert venue that killed at least 137 people.

The attack, which has been claimed by an affiliate of the Islamic State group, was the deadliest on Russian soil in years. Russian authorities arrested the four suspected attackers Saturday, with seven more people detained on suspicion of involvement in the attack.