Profile of a terror group: ISIS-K, the group claiming responsibility for the Moscow concert hall attack?

The Islamic State in Khorasan (ISIS-K) claimed responsibility for Friday’s deadly massacre at a concert hall in Moscow, releasing graphic footage of the attack.

Three days after ISIS-K, an offshoot of the Islamic State, claimed responsibility for an attack that killed more than 130 people — one of the worst terror attacks in Russia in decades — Russian …