The BBC's Nick Bryant: "The horror of what happened that day will be impossible to forget"

In April, two teenage gunmen went on the rampage at the school, killing 12 of their fellow students and a teacher before taking their own lives.

CBS aired clips of the videotape on Tuesday, which showed one of the gunmen opening fire in the Columbine cafeteria.

Barbara Monseu, the school district administrator in Jefferson County, Colorado, said school officials had tried unsuccessfully to stop the tape from being broadcast.

The silent, grainy video shows students bustling around the cafeteria at around lunchtime.

Students scatter

Students run away during evacuation of the school

It then shows what appears to be a gunman diving for cover moments before one of his bombs explodes.

Students are seen frantically scattering and colliding as they try to duck under tables for safety.

Later, a second gunman appears, kneels on the floor, steadies himself and opens fire on what is believed to be a huge propane bomb.

Authorities said no bodies were shown in the snippets aired on television.

Bad for the families

The clips had only previously been shown at a training seminar to help school officials prepare for bomb threats.

Colorado authorities say they were outraged by the decision to air the footage and tried to stop it going out.

The Jefferson County sheriff's department, which had kept a copy of the videotape, said no authorisation had been given for it to be aired.

"The bad thing is for the victims' families," said spokesman Steve Davis. "They are being notified as we speak that some stations have chosen to air this."

One station defended its decision by arguing the footage had news value. Another said it was up to the viewers to decide whether to watch the footage or not.