President Vladimir V. Putin vowed to punish those responsible for the assault, one of the deadliest in Russia in decades. U.S. officials have attributed the attack to ISIS-K, a branch of the Islamic State that has been active in Iran and Afghanistan.President Vladimir V. Putin vowed to punish those responsible for the assault, one of the deadliest in Russia in decades. U.S. officials have attributed the attack to ISIS-K, a branch of the Islamic State that has been active in Iran and Afghanistan.