Moscow, Russia, shooting attack at a concert hall

ISIS, also known as the Islamic State, claimed responsibility for Friday's deadly attack on a concert in Moscow, releasing video footage showing its gunmen carrying out the worst terror attack Russia has seen in decades.

Here's what we know about the group:

When was it formed?: ISIS Khorasan was formed in 2015 and is active in Afghanistan, Pakistan and Iran. It is an offshoot of ISIS, the terrorist group that emerged in Syria and Iraq and, at its peak, controlled a large swath of territory. Five years after the fall of the caliphate declared by ISIS in Iraq and Syria, the organization has transformed into a terrorist network with cells spread throughout the world, including Africa, the Middle East, Central Asia, and Southeast Asia.

What is the extent of his connection to ISIS? The relationship between the groups is not entirely clear. The subordinate groups share a common ideology and tactics, but the depth of the relationship between them – such as the chain of command and control – is never fully established.

What is his belief?: Like its parent group, ISIS-K aims to create a “pure Islamic state,” according to the Center for Strategic and International Studies (CSIS) — which describes the group’s vision of a “global, transnational caliphate” governed by Islamic law.

Why attack Russia?: Several experts told CNN that ISIS has a long-standing animosity against Russia and Putin. “Russia has been at or near the top of the ISIS list for many years,” said Daniel Byman, director of the Security Studies Program at Georgetown University.

Read more about ISIS-K is here.