The auditorium at Crocus City Hall was about three-quarters full, with the crowd waiting to see Picnic, a band popular since the Soviet days of the early 1980s. But the concert was sold out in the 6,200-seat hall, so some of the audience was still likely getting food or were shedding their heavy coats in the cloakroom.

It was 7-10 minutes before the start of the show, scheduled for 8 p.m., said concertgoer Dave Primov.

Then came the popping sounds.

Initially I thought: fireworks or something like that Primov told The Associated Press. I looked at my colleague, and he also said: Fireworks, probably.