Police confirmed yesterday that they had recovered the video "within hours" of the shooting. Lt-Gen John Kiekbusch of Jefferson County police told CNN the film contained "strong indications" that the two boys had "walked through or enacted that which they eventually committed".
According to one classmate, Eric Harris and Dylan Klebold, the two students who killed 12 of their classmates and a teacher before turning their guns on themselves, made the video last autumn with thescenes that proved an eerie foreshadowing of Tuesday's tragic events.
"It was disturbing to everyone who saw it," said the student, Chris Reilly.
Police also stumbled upon at least two huge bombs in the school kitchen, suggesting that more than two students were involved and there might have been a plan to blow up the buildings.
Initial reports suggested the explosive devices were 30- gallon propane tanks with triggers taped to them. Investigation teams hurried out of the high school, in Littleton, Denver, to allow bomb squads to return to the campus and neutralise the devices.
John Stone, the sheriff of Jefferson County responsible for law enforcement at the school, said investigators were searching for possible accomplices of Harris and Klebold.
"I can't believe they were capable of doing this by themselves," he said after briefing the US Attorney General, Janet Reno, who was in Littleton.
It was not clear if the propane tanks were already in the school, or if they were brought in by the attackers. It was also not clear why bomb squads had not discovered the devices in their search of the school after the shooting ended.
A friend of Harris and Klebold told a television station he thought authorities were interested in at least one member of the "Trenchcoat Mafia", the school clique fascinated by guns, violence and death. Pat McDuffee said he thought the police were pursuing Chris Morrison, whom he described as the most eccentric of the group. He said the whole group was familiar with explosives.
Four students were detained after the shooting, but were later released. There have been no arrests.