The Islamic State group has claimed responsibility for a gun attack at a rock concert in Moscow on Friday, killing at least 40 people, wounding 100, and setting off an inferno in the theatre. Attackers dressed in camouflage uniforms entered the building, opened fire and threw a grenade or incendiary bomb, according to a journalist for the RIA Novosti news agency at the scene. Fire quickly spread through the Crocus City concert hall in the Krasnogorsk suburb in the north of the Russian capital, which can hold several thousand people and has hosted top international artists. “According to prelim…