Two teenagers in black trench coats laughed and hooted as they opened fire on classmates in their suburban Denver high school, killing 16 people, including themselves, in the deadliest U.S. school rampage on record.

Eric Harris (18) and Dylan Klebold (17) marched into the library of Columbine High School yesterday with guns and pipe bombs, demanding that "all jocks stand up. We're going to kill every one of you," said student Aaron Cohn.

One of them looked under a desk in the library and said "Peek-a-boo," then fired, Cohn said. Anyone who cried or moaned was shot again. One girl begged for her life, but a gunshot ended her cries.

Cohn said one killer put a pistol to his head but did not shoot him. Instead, he said, the shooter turned his attention to a black student, saying, "I hate niggers." Cohn heard three shots but couldn't see what happened.

The bodies lay crumpled in the schoolhouse where they were shot; officers delayed removing the corpses until later today because of possible booby traps.

Bombs were found in and around the school, including in two cars in the school parking lot. More than 11 hours after the shootings, a bomb on a timer blew up, but no one was injured.

At least 23 people were hospitalized, most of them with gunshot wounds. One girl suffered nine shrapnel wounds. At least nine victims were in critical or serious condition; one was in guarded condition.

The dead included students and at least one faculty member, said Superintendent Jane Hammond. Students and Denver media identified the gunmen as Eric Harris and Dylan Klebold, both juniors at the school. The two exchanged shots with police and were later found dead in the library with self-inflicted gunshot wounds and bombs around their bodies, said sheriff's spokesman Steve Davis.

"It appears to be a suicide mission," said Sheriff John Stone. While the motive was unclear, several students said Harris and Klebold were members of a group calling itself the "Trenchcoat Mafia," outcasts who bragged about guns and bombs and hated blacks and Hispanics, as well as athletes. Police who searched Harris' home said they found bomb-making material. Harris and Klebold both had juvenile records in Jefferson County District Court, according to a district court clerk - they were implicated in a 1998 criminal trespass case. Klebold received a deferred sentence.

Sixteen-year-old Matt Good told the Denver Post that during the past year Harris had started wearing all black, spoke German, and was obsessed with anything German or about the Second World War. The local sheriff noted that yesterday was Adolf Hitler's birthday.

But some neighbours described Harris and Klebold as quiet boys - one said Klebold was a gifted and talented student who also took part in a fantasy baseball league. They said the Harris family moved in about two years ago and that the boy's father, Wayne, recently retired from the military. Harris may have taken his father's guns for the massacre, they said.

Littleton, a middle-class suburb of Denver with a population of 35,000, is the latest American community shocked by school violence. Since 1997, shootings have led to calls for stricter laws, tighter security and closer monitoring of troubled students. Two people were killed in an attack at a school in Pearl, Mississippi; three in West Paducah, Kentucky; five in Jonesboro, Arkansas, and two in Springfield, Oregon.

In Washington, President Bill Clinton's reaction was muted. He said intended to wait for the facts to emerge before taking any new steps to protect children in US schools. 'You know what we put out before,' he told reporters. 'You know the efforts we've made. And I just think that tonight we need to focus on the families that lost their kids, on the children that are wounded, on the grieving of the community.'

'We do know that we must do more to reach out to our children and teach them to express their anger and to resolve their conflicts with words, not weapons. And we do know we have to do more to recognize the early warning signs that are sent before children act violently,' he added.

The two gunmen, wearing fatigues and ankle-length black coats, opened fire in the parking lot around 11:30 a.m. MDT (1730 GMT) before entering the school cafeteria. Bullets ricocheted off lockers as students raced toward the exits.

Student Nick Foss saw one of the gunmen pull a sawed-off shotgun from under his coat. "Finally I started figuring out these guys shot to kill for no reason," Foss said. The gunman "didn't say anything. When he looked at me, the guy's eyes were just dead."

The killers fired as they walked into the cafeteria and continued shooting as they moved upstairs into the library. Cohn said he heard one girl begging for her life and then a bang. "They were laughing after they shot. It was like they were having the time of their life," he said.

Dozens of students hid in classrooms before escaping with the help of police in an armoured car. Others were trapped for hours while police commando teams searched for the gunmen and their bombs. Some students had called their parents on cellular phones from inside the building to whisper they were OK.

Hundreds of police officers from throughout the Denver area surrounded the school, which has 1,800 students. As TV images of the scene were broadcast nationwide, helicopter ambulances used a sports field as a landing pad, and officers in helmets and camouflage gear took cover behind squad cars.

Students who fled the building wept and held their hands above their heads while police frisked them. Many parents gathered at a nearby elementary school and answered questions from investigators, who were trying to identify some of the victims based only on what they were wearing.