Iran tipped off Russia about the possibility of a major “terrorist operation” on its soil ahead of the concert hall massacre near Moscow last month, three sources familiar with the matter said.

In the deadliest attack inside Russia in 20 years, gunmen opened fire with automatic weapons at concertgoers on March 22 at the Crocus City Hall, killing at least 144 people in violence claimed by the Islamic State militant group.

“Days before the attack in Russia, Tehran shared information with Moscow about a possible big terrorist attack inside Russia that was acquired during interrogations of those arrested in connection with deadly bombings in Iran,” said one source.