Former students Eric Harris, 18, and Dylan Klebold, 17, shot themselves after rampaging with guns and bombs at Columbine High School in Littleton, Denver.

The pair, who reportedly belonged to school clique called the "Trenchcoat Mafia", wore long black coats and masks and laughed repeatedly as they shot dead 12 students and one teacher.

Stephen Sackur reports on a "suburban high school turned into a killing field"

Initially it was thought that up to 25 people had died.

It is the United States' worst school massacre. There has been a spate of similar killings over recent years.

US President Bill Clinton used a speech to White House volunteers on Wednesday to express national shock at the tragedy.

Eye witnesses from the school give their account of the scene

"We have to take this moment once again to hammer home to all the children of America that violence is wrong, and to show children by the power of our own example how to resolve conflicts peacefully," he said.

Police moved slowly through the school after one bomb with a timer exploded and as many as 30 others were found. No-one was hurt in the explosion.

Devices including pipe bombs, timed explosives and improvised hand grenades were discovered near and around the suspects' bodies.

One bomb was rigged to the petrol tank of a car in the school car park.

The gunmen opened fire at about 1130 local time (1730GMT), and were said to have targeted crowded areas, such as the canteen and library.

Screaming pupils tried to hide as the two teenagers singled out "jocks" [athletes] and, in some reports, ethnic minorities, before randomly firing down corridors.

At least 28 people were injured and some remain in a critical condition. A girl is said to be in a "stable" condition after suffering nine gunshot wounds to the chest.

During the massacre, one blood-spattered pupil fell from a window into the arms of police officers. Another held a sign to a window which read: "Help me, I'm bleeding to death".

Jeff Schwartz from the Swedish Medical Centre: "We have a little bit of good news"

Following reports of gunfire and explosions, more than 200 heavily-armed police from throughout the Denver area surrounded the school and guided terrified pupils out of buildings.

Police are understood to have traded shots with the two teenagers, before they killed themselves. Their bodies were found in the library.

Pupils help their wounded classmates

A spokesman for the Jefferson Police Department said the pair were "on a suicide mission". The killers' group, the Trenchcoat Mafia, were reportedly social outcasts and alienated from other pupils.

Pupils at the school described the suspects as "militaristic", and there have been unconfirmed reports that one of the gunmen posted a warning about the massacre on his "hate-filled Website".

An unnamed friend of the gunmen told BBC News that they had "joked and bragged" about killing.

"I never thought that they'd do it. I never thought they'd go through with it," he said.

Three people, believed to be friends of the suspects, were arrested in a field near the school and led away in handcuffs.