Who are ISIS-K and why would they attack a concert hall in Russia?

The U.S. has intelligence confirming Islamic State's claim of responsibility for a deadly shooting attack at a concert near Moscow on Friday, a U.S. official told Reuters.

Here is information about the Islamic State's Afghan branch known as ISIS-K and their motives for attacking Russia:

WHAT IS ISIS-K?

Islamic State Khorasan (ISIS-K), named after an old term for the region that included parts of Iran, Turkmenistan and Afghanistan, emerged in eastern Afghanistan in late 2014 and quickly established a reputation for extreme brutality.