The Islamic State group, also known as ISIS, claimed responsibly 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.

US 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 US has received a stream of intelligence that ISIS-K was determined to mount an attack in Russia, sources told CNN, and passed those warnings on to Moscow. US Vice President Kamala Harris said Sunday that “what we know to be the case is that ISIS-K is actually, by all accounts responsible for what happened.”

Here’s what we know about ISIS-K.

Who are ISIS-K?

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.

The connection between ISIS-K and its apparent parent group 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.

By 2018, ISIS-K was ranked the world’s fourth-deadliest terror group, according to the Institute for Economics and Peace, which monitors global terrorism.

The Taliban’s takeover of Afghanistan in August 2021, and the withdrawal of US troops from the country, thrust ISIS-K into the global spotlight – especially after the group orchestrated a deadly bombing outside Kabul airport that killed 13 US military personnel and 170 Afghans.

It was ISIS-K’s most globally consequential action to date and drew a promise of retribution from US President Joe Biden.

The United Nations estimated in 2021 that the group has up to 2,200 core fighters based in Afghanistan’s Kunar and Nangarhar provinces.