Dozens of people were brutally murdered by Islamic terrorists at a concert hall in Russia’s capital city on Friday night in scenes that went viral on social media.

The attack, which was the deadliest terror attack on Russian soil in over a decade, happened at a sold-out concert at Crocus City Hall as the rock band Piknik was set to begin performing.

At least four men wearing camo began massacring people outside with fully automatic weapons in highly disturbing scenes that were caught on video. At least 40 people were killed and more than 140 injured, with those numbers likely to rise.

The attack came two weeks after the U.S. warned Americans in Moscow to avoid large public gatherings — specifically concerts — because terrorists were planning an attack. The New York Times said that the warning was directly related to the attack that unfolded and that U.S. officials specifically reached out and warned the Russians.

ISIS took responsibility for the terrorist attack, saying in a statement that they attacked “a large gathering of Christians,” causing hundreds of casualties before fleeing the scene.

The U.S. Intelligence Community collected intelligence in March indicating that a branch of ISIS was planning a terrorist attack in Moscow, which led to the U.S. State Department issuing a warning earlier this month urging Americans to avoid large crowd.

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“ISIS-K has been fixated on Russia for the past two years,” Colin P. Clarke, a counterterrorism analyst at the Soufan Group, told The New York Times, noting that the terrorist group has focused on attacking Putin in propaganda videos. “ISIS-K accuses the Kremlin of having Muslim blood in its hands, referencing Moscow’s interventions in Afghanistan, Chechnya and Syria.”

The U.S. warned Iran about an ISIS terrorist attack back in January that the terror group later successfully executed when it carried out suicide bombings in the southeastern town of Kerman, killing 84 people with hundreds more injured.