Distraught survivors poured out of the building under the protection of hundreds of heavily-armed police officers after two former pupils in masks and black trench coats used guns and explosives to create carnage in the school.

A student identified only as Janine described how she and her fellow pupils looked on in disbelief as the killing started.

Eyewitness: "There was blood everywhere"

"We didn't think it was real and then we saw blood," she said.

"There was a guy at a table right next to us and they just shot him. They just walked away and he was sitting there in a pool of blood," said another girl.

The two gunmen, identified by pupils as Eric Harris and Dylan Klebold, reportedly fired on crowded areas like the canteen and library.

Four witnesses describe the shooting

"I was in the library and the whole room was filled with smoke. I heard gunshots all over and the two guys ... came into the library and started shooting anyone that they could," said a girl who hid beneath a table to escape the hail of bullets.

Student Sarah Lammers said the fire alarm went off and students began to leave the classrooms.

"We went out in the hall and another teacher yelled at us to come back in because there were men with guns. I heard about 10 shots right outside my door. And there was also a bullet that came through the wall," she said.

"We were just scared because we knew that they weren't afraid to shoot anybody ... we knew that they would shoot us right away."

Many of the survivors were stunned by the callousness of the gunmen as they picked their targets.

A hysterical girl recounted how she pleaded with the men for her life.

"Everyone around me got shot. I begged him for 10 minutes not to shoot me. He just put the gun in my face and there was bleeding everywhere and he started laughing and was saying that it was all because people had been mean to him last year."

Student Aaron Cohen said: "They were laughing after they shot ... it was like they were having the time of their life."

Joshua Lapp, who hid beneath his desk in the library, said: "They'd shoot somebody, they'd laugh, they'd giggle ... you'd hear a shot go off, you'd hear somebody yell and scream, another shot go off and they'd yell and scream, another shot and there would be silence."

Witnesses described how the two men picked on "jocks" [athletes] and may also have targeted black and Asian students.

One pupil described how they shouted: "All jocks stand up. We're going to kill every one of you."

'It's not looking good'

Many parents were still waiting for news of their children 18 hours after the shooting. They were asked to bring dental records to help in possible identification.

The killers are believed to have selected their targets

One man suffered what appeared to be a heart attack, while other parents were physically sick with worry.

Michael Shoels had still not received news of his son Isaiah at 2000 local time. He said he feared his son may have been shot because he was black.

"This late, it's not looking good at all. It's like a Harris dream I'm trying to wake up from ... I just wish everybody would pray for my family,'' he said.

Police confirmed that the suspected gunmen had killed themselves following the massacre, but efforts to clear the school building were being hampered by explosive booby-traps left around the school.

More bombs found

Speaking after the killings, Sheriff John Stone said. "It appears to be a suicide mission. I think we've got a lot of media attention on these things around the country and you don't know how much that plays into giving somebody else an idea to do it."

He added: "The bomb guys keep finding more and more of this stuff ... [they] will probably be working all night long ...The problem is we don't want to have one go off that we've overlooked."

Up to 23 people are known to have been injured in the killing spree.

Spokesman for the local Swedish Hospital in Denver, Jeff Schwartz, paid tribute to the emergency services operation to save the lives of the victims.