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 Soviet-era 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 143 people were killed and dozens more injured in the deadliest attack in Russia since the 2004 Beslan school siege. Islamic State claimed responsibility for the attack.

The Federal Security Service (FSB) said 11 people - including the four alleged attackers - had been detained in the Bryansk region, about 340 km (210 miles) southwest of Moscow, as they headed for the border over which they hoped to escape to Ukraine. Kyiv has denied any involvement in that attack.

CAMOUFLAGE AND COMBAT VESTS

The men, clad in camouflage and with combat vests containing dozens of spare magazines, arrived at Crocus City Hall at about 7:40 p.m. (1640 GMT) in a minivan, leaping out of the back door and heading towards the entrance with their weapons, witnesses said.

They fired directly through the glass doors of the main entrance, shooting at anyone who crossed their path. Dozens of bodies, some in pools of blood, lay motionless on the marble floors and at the main entrance.

Some people smashed through reinforced windows and locked exits with their hands as shots echoed around 14-year-old hall just 20 km (12 miles) west of the Kremlin.

After shooting people at the entrance, the men made their way into the hall itself just as hundreds of people were taking their seats for the concert which had been sold out.

"Some thought it was a kind of special effect of some sort," one witness, Anastasia Rodionova, told Reuters. "Then I saw with my own eyes how people were dropping and the automatic gun fire began."

"Your instinct for self-preservation kicks in, your eyes widen, where can I run? Then someone shouted to us: get up - don't lie down or they will shoot us all right now."

Rodionova said some men were able to smash down a door to the street and escaped.

Loudspeakers began to blare out that the concert was being cancelled for "technical reasons" and that people were requested to the leave the hall.

RUSH TO ESCAPE

Verified video showed people rushing for the exits as repeated gunfire echoed above screams. The attackers walked through the concert hall aiming and then firing at civilians with controlled bursts.

"They started firing at us. I fell down onto the floor," one injured woman told BRIEF media from a Moscow hospital bed. She crawled to the exits. "A girl next to me was killed."

Some ran to escape the men. Others cowered behind maroon seats. One woman said she told her friend to lie down behind the seats as the gun fire got louder and louder.

Russian investigators said the men began to set fire to the building. Some witnesses said they poured some sort of liquid on seating and curtains in several places before igniting it.

Witnesses told of leaping over fire, some with their clothes melting, to escape the blaze which swiftly spread over an area of 12,900 square metres, sending flames and a plume of black smoke billowing into the night sky.

"They were shooting. We were in the far corner," Andrei, one witness, told Reuters at the scene. "We went down the fire escape routes to the back."

The roof collapsed and hundreds of firefighters battled for hours to contain the blaze which gutted the entire hall. All that was left were the charred iron support beams and the steel frames of hundreds of seats.

The Baza Telegram channel, which is known for its close contacts with Russian special services, said 14 bodies were found on evacuation staircases and 28 bodies had been found in one of the toilets.

The bodies of whole families were found, dead mothers embracing their dead children.