Warning: Some may find the details in this story disturbing.

(NewsNation) — Islamic State-affiliated Amaq news agency released graphic footage Saturday showing gunmen opening fire in a Moscow concert hall that left scores dead.

The footage circulated widely on social media, showing gunmen roaming through the lobby area of Crocus City Hall as bodies lay on the ground. Voices were distorted and faces blurred, but one gunman was seen shooting a person at close range while another repeatedly slashing a man’s neck lying on the floor.

The Islamic State group claimed responsibility for the attack that killed at least 133 people, the most deadly attack in Russia in years.

Much remains unknown about the Friday night attack, including whether it related to a security alert the U.S. Embassy in Moscow issued two weeks earlier and whether it signals a resurgence of the group in the West.

Russia continues to investigate after detaining 11 suspects but it wasn’t possible to confirm the authenticity of statements issued by Russian investigators.

The attack was the latest in a long series of bombings and sieges that have unsettled and outraged Russians during Putin’s nearly quarter-century as either prime minister or president.

The Associated Press contributed to this report.