Denver, Colorado - Officials in the western American state of Colorado revealed on Thursday that police had had numerous run-ins with teenage assassins Dylan Klebold and Eric Harris before they slayed 13 people at Columbine High School in 1999.

Klebold and Harris, aged 17 and 18 respectively at the time of their deaths, burst into the school in Littleton, Colorado, on April 20, 1999 and killed 12 students, a teacher and themselves, firing off more than 200 rounds of ammunition.

The revelation that police had a total of 15 run-ins with the two killers up to two years before the school shootings that shocked America, reignited relatives' anger during a press conference on Thursday.

"Today is a very important milestone in addressing the tragedy of Columbine," Colorado Attorney General Ken Salazar said in a statement.

'My hope is that these efforts will shed light on the specific events leading to the shootings'

Salazar's office released a report investigating a 1997 complaint made to the Jefferson County Sheriff's Office regarding Harris.

At the same time, the authorities had unveiled evidence from the school massacre including guns, projectiles and furniture from the school cafeteria where many of the victims were slain.

"My hope is that these efforts will shed light on the specific events leading to the shootings and the lessons to be drawn for the future for family awareness, school safety, counselling of children, and related matters," Salazar said.

The report did not fault the police for not taking action against the teens prior to their shooting spree.

The police contacts initially involved seemingly innocent student pranks such as snowball throwing and joke phone-calls, but "progressed" to threats over the Internet and breaking into a van.

Harris first came to police attention in 1997 after complaints he had been lobbing snowballs at a car and had made prank telephone calls.

Shortly afterwards, police discovered an Internet page frequented by Harris that contained information on explosives.

Prior to the killings, the local community had viewed Klebold and Harris as intelligent adolescents from comfortable family backgrounds. - Sapa-AFP