Islamic State, the militant group that once sought control over swaths of Iraq and Syria, claimed responsibility for the attack, the group's Telegram channel said.
The United States has intelligence confirming Islamic State's claim of responsibility for the shooting, a U.S. official said on Friday. The official said Washington had warned Moscow in recent weeks of the possibility of an attack.
"We did warn the Russians appropriately," said the official, speaking on condition of anonymity, without providing any additional details.