LAKEWOOD, Colo. -- Colorado authorities on Thursday released graphic evidence connected to the 1999 Columbine High School shooting that killed 12 students and a teacher in April 1999.

Jefferson County (Colo.) sheriff's officials publicly unveiled a display of the murder weapons, bullet fragments, the chairs and tables where the victims were gunned down, angering friends and families of the victims.

COLUMBINE SHOOTINGS Release Evidence? Slideshow: April 20, 1999

A message board put up in a school window that day still says in blue Magic Marker, "1 bleeding to death."

The victims' families were allowed to see the items first Wednesday afternoon. The experience dredge up painful memories.

"The bullet holes in the seats ... bullet holes everywhere. Just to see that. I'm sorry. I'm sorry guys," said a tearful Betty Shoels, the aunt of Isaiah Shoels, one of the students killed in the shootings.

But perhaps more chilling was video released Thursday to the public.

Authorities released two videos, one of the scene in a park across the street from the school on April 20, 1999, and another, a 94-minute compilation of videos made by the killers, Eric Harris and Dylan Klebold.

There is some evidence that officials will not release, which includes the school's own investigation into the massacre and a deposition given by Wayne Harris, the father of one of the killers.

Also on Thursday, Colorado Attorney General Ken Salazar said authorities had at least 15 contacts with Harris and Klebold dating back two years before their murderous attack.

Salazar said an investigation is under way into whether Jefferson County sheriff's officials covered up what they knew about Harris and Klebold, who took their own lives after the murders at the high school near Littleton, Colo.

Asked if he thought there was a cover-up, Salazar said he didn't know. He also said he's found no evidence of negligence so far.

Salazar did not blame the Jefferson County sheriff's office for missing warning signs about Harris and Klebold.

Instead, Salazar summarized how investigators reacted to 1997 complaints about Harris, from a thrown snowball that cracked a car window to a prank telephone call.

There were more ominous signs too. Authorities had found a Web site run by Harris.

On it, the teens had built pipe bombs and concluded, "Now our only problem is to find the place that will be ground zero." The site also included a death threat against a fellow student.