Iran alerted Russia about security threat before Moscow attack. In the deadliest attack inside Russia in 20 years, gunmen opened fire with automatic weapons on concertgoers March 22 at Crocus City Hall.
At least 144 people were killed in an act of violence attributed to the militant group State Islamic. The United States had also warned Russia in advance about a possible Islamist attack, but Moscow downplayed the warning. However, it is harder for Russia to ignore intelligence from its diplomatic ally Iran about the attack, which has also raised questions about the effectiveness of Russian security services.