Lakewood, Colo. ? Law officers were warned about the Columbine High killers at least 15 times in the two years before their murderous rampage in 1999, the state attorney general said Thursday in a report that outraged the families of the victims.

Colorado Atty. Gen. Ken Salazar said he was still digging for details on whether authorities might have missed possible warning signs about the killers. Asked if he thought there was a cover-up, Salazar said: “I do not know today.”

Salazar’s report angered families of the 13 victims because it did not blame the Jefferson County sheriff’s office for missing warning signs about Eric Harris and Dylan Klebold.

Some wept as Salazar concluded he had found no sign of negligence, and families said his narrowly tailored report did nothing to ease their anguish.

“It’s done. You know it. They’re not going to do any more,” Randy Brown said as he hugged his sobbing wife. The Browns told the sheriff’s office that Harris was making death threats against their son in August 1997.

While Salazar detailed his report, some 900 people lined up to view a vast and chilling display of evidence collected in the case. Authorities also released a 90-minute compilation of videos made by Harris and Klebold.

Wearing trenchcoats and sunglasses, the two are seen stalking through the hallways of Columbine just five months before the attack, portraying hitmen offering their services to students victimized by bullies. At one point, the two roar obscenities into the lens and promise a brutal death for their targets.

Harris, 18, and Klebold, 17, killed 12 students and a teacher before killing themselves on April 20, 1999, at the school near Littleton. Family members have long said the sheriff’s office, the gunmen’s parents and others missed signs that the teens were capable of murder.