Russian media outlets reported that between two to five assailants were involved in the attack and also used explosives, causing a massive blaze at the Crocus City Hall on the western edge of Moscow. The attack took place as crowds gathered for a concert by Picnic, a famed Russian rock band, at the hall, which can accommodate over 6,000 people. Russian media reports said visitors were being evacuated, but some said an unspecified number of people could have been trapped by the blaze