ISIS, also known as the Islamic State group, 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.

Here’s what we know about the group:

When was it formed?: ISIS-K was formed in 2015 and has been active in Afghanistan, Pakistan and Iran. It is a branch of ISIS, the terror group that emerged in Syria and Iraq and, at its peak, controlled a huge stretch of territory. Five years since the fall of ISIS’ self-proclaimed caliphate across Iraq and Syria, the group has morphed into a terror network with cells spread around the world, including in Africa, the Middle East, Central Asia and Southeast Asia.

How is it linked to ISIS? The connection between the groups is not entirely clear. The affiliates share an ideology and tactics, but the depth of their relationship – such as the chain of command and control – has never been fully established.

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

Why attack Russia?: ISIS has a longstanding animosity against Russia and Putin, several experts told CNN. “Russia has been at the top or near the top of the list of ISIS for many years,” said Daniel Byman, director of Georgetown University’s security studies program.

Read more about ISIS-K here.