Who are ISIS-K, the group linked to the Moscow concert hall terror attack? [Video]

The Islamic State group, also known as ISIS, claimed responsibility for Friday’s deadly assault on a concert venue in Moscow, releasing graphic footage purporting to show its gunmen carrying out what was Russia’s worst terror attack in decades.Russian authorities have accused four men from the former Soviet republic of Tajikistan of being behind the attack, which left at least 137 people dead and more than 100 injured. The suspects, who are charged with committing a terrorist act and face possible life imprisonment, have been remanded into pre-trial detention through May 22 after appearing in court in Moscow on Sunday.U.S. officials have tied the attack to the Islamic State in Khorasan, or ISIS-K, an ISIS affiliate that operates in central Asia, which has become one of the region’s most brutal and feared terror groups.Since November, the U.S. has received a stream of intelligence that ISIS-K was determined to mount an attack …