Within hours, IS claimed the attack and linked it to an affiliate called the Islamic State in Khorasan, or IS-K. They subsequently made public graphic video of the perpetrators shooting at the concertgoers, which the BBC has confirmed to be authentic.
Russian authorities have insisted, without providing any proof, that the assailants were helped by Ukraine and were in the Bryansk region getting ready to cross the border when they were apprehended. President of Ukraine Volodymyr Zelensky denied these allegations on Sunday, describing as "absurd" the idea that the guys were trying to pass a highly mined border brimming with Russian soldiers. Seven more people were detained in Russia on accusations of supporting the attack.
Earlier this month, the US had alerted Moscow to the possibility of a major strike and then sent a public warning to its nationals there. The warning was written down as propaganda and an attempt to tamper with its presidential contest by the Kremlin. Washington declared after the attack that there was no cause for doubting the IS claim.
Targeting Russia or its interests overseas is nothing new for IS and its supporters. The organisation took credit for the 2015 Russian jet attack over Egypt, which killed 224 people, the most of them being Russian nationals. They also cited a 2017 bombing on the St. Petersburg subway that lost fifteen lives.
For a number of reasons, according to security experts, including Russia's part in destroying IS's stronghold in Syria while preserving President Bashar al-Assad's rule, Moscow's two bloody conflicts in Muslim-majority Chechnya from 1994 to 2009, and the Soviet-era invasion of Afghanistan, IS views Russia as a major target.
One of the most capable, IS-K is named after a historical designation for the area and works mostly in Afghanistan and portions of Central Asia.