The four armed men walked calmly towards the metal detectors at Crocus City Hall, firing their automatic weapons point-blank in short bursts at terrified civilians who fell screaming in a hail off bullets.

Nearby, one witness named Natalya had just taken off her coat and was standing in line on Friday evening at the internal entrance to the 6,200-seat concert hall outside Moscow, where rock group “Picnic” was to perform its hit Afraid of Nothing.

“The shots came from behind us,” Natalya, who asked for her surname not to be used, told Reuters. She was just about to enter the stalls.

“It was loud, like a firecracker blast, fireworks, but like an automatic burst. I could hear it right behind me, not far away,” Natalya said.

Then Natalya ran for her life.

“Everyone was screaming; everyone was running,” she said. She ran to the nearby metro station through the cold Moscow night without her coat and escaped. “I experienced terrible emotions. It is simply a nightmare.”

Reuters was able to piece together some of what took place at the concert hall from interviews with witnesses, video footage from the scene and Russian official accounts and media reports. More than 133 people were killed and dozens more injured in the deadliest attack in Russia since the 2004 Beslan school siege. The ISIS-K claimed responsibility for the attack.

The Federal Security Service said four alleged attackers had been detained in the Bryansk region, about 340 km southwest of Moscow, as they headed for the border over which they hoped to escape to Ukraine.

The men, clad in camouflage and with combat vests containing dozens of spare magazines, arrived at Crocus City Hall at about 1640 GMT in a minivan, leaping out of the back door and heading towards the entrance with their weapons, witnesses said.