The exhibition was organised by police to prove to victims’ families that they were not hiding anything. Families were also told that the killers had 15 run-ins with the law before gunning down 12 pupils and a teacher in America’s deadliest school shooting.

Eric Harris, 18, and Dylan Klebold, 17, first caught the law’s attention for pranks like pelting cars with snowballs, a new report by Colorado attorney general Ken Salazar found. But incidents escalated into online death threats and bragging they had pipe bombs and were seeking a “ground zero”.

Mr Salazar’s investigation was sparked by the chance discovery, last October, of a 1997 file in a police manual. The file recorded an anonymous tip-off about the website. Officers did not act on it and a search warrant for Harris’s home issued following the later discovery of a detonated pipe bomb in a park was never served.

Mr Salazar is now trying to trace the missing police record of this incident.

Officials also presented an ominous video - Hit men for hire - made by the students for a class project - and other chilling exhibits. These included a note reading, “Bleeding to death”, written by chemistry teacher Alan Cram in a vain effort to direct police to dying colleague Dave Sanders.

“The idea is to get as much information out as possible to dispel suspicions that the sheriff is hiding anything,” said Colorado’s deputy attorney general Ken Lane.

Mr Lane said the inquiry was concerned only with certain police files, not allegations that officers were negligent. “On the question of whether searching the home and arresting Harris and Klebold would have prevented Columbine, I don’t know if there’ll ever be an answer,” he said.

However, Randy Brown, who told police about the website in 1998 and whose sons narrowly survived the massacre, said officers missed warning signs.

“We want them to say: ‘You know what, we screwed up’,” he said.