The Backstory on ISIS-K, the ISIS Affiliate Believed Responsible for Moscow Attack

The suburban Moscow concert hall assault last Friday that killed more than 130 people was the deadliest attack inside Russia in 20 years. It raised major questions about the capabilities and ambitions of the terror group that U.S. officials believe was responsible: ISIS-Khorasan, or ISIS-K, an affiliate of the self-declared Islamic State that emerged in Afghanistan.