After learning no one would be blamed for missing possible warning signs that foreshadowed the Columbine High School (search) massacre, Judy Brown simply wept.

"It's done. You know it. They're not going to do any more," Randy Brown said as he hugged his sobbing wife.

• Raw Data: Columbine Report (pdf)

The Browns were among a dozen relatives of Columbine victims who stood disconsolately against the back wall of the cavernous fairground auditorium Thursday after Attorney General Ken Salazar said the Jefferson County sheriff's office was not at fault for failing to follow up on warnings about Eric Harris (search) and Dylan Klebold (search).

Asked if he thought there was a cover-up later, Salazar said: "I do not know today." And Friday, he said his investigation wasn't over.

"My conclusion with respect to the April 20, 1999, time frame, before the school shooting itself, there was no cover-up," Salazar said on CBS's "The Early Show." "We are now in the process, however, of looking at activities that occurred after April 20, 1999."

Brown, her two sons and husband said they had tried repeatedly to convince authorities that Harris was dangerous, telling the sheriff's office in August 1997 that Harris was making death threats against their son, who was not injured in the attack.

Salazar said the sheriff's office had 15 dealings with the teens — from dispatch reports to official investigative reports — stemming from six separate incidents, including a January 1998 arrest while breaking into a van, and complaints of threats and vandalism. Salazar made no conclusions about what investigators could have done differently.

"They did do an investigation," he told CBS. "There was some follow-up made and there were reports taken. Now, it didn't go as far as it should have gone, now that we're looking at it in hindsight."

Salazar said the investigation wasn't complete because one key report tied to a search warrant was still missing, but he made clear Thursday he would not be providing the answers the families sought.

While Salazar detailed his report, nearly 1,000 people lined up to view a vast and chilling display of 10,418 separate pieces of evidence in the case — from the murder weapons and trench coats to bullet fragments and the chairs and tables where people died. There were 13 body bags.

Erin Walton, 20, wept as she looked at a message board that said in blue Magic Marker: "1 bleeding to death," referring to mortally wounded teacher Dave Sanders. Walton was there that day, trying to stanch his wounds with a sweatshirt.

"I came here for closure, and out of curiosity," Walton said.

The evidence was on display for a single day. Much of the material is headed for thee state archives, along with the video material shot by Harris and Klebold.

Authorities also released a 90-minute compilation of videos made by Harris and Klebold.

Wearing trench coats and sunglasses, the two are seen stalking through the hallways of Columbine five months before the attack, portraying hit men 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.

"I don't care what you say; if you ever touch him again, I will fricking kill you," a wild-eyed Harris screams on the tape.

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

Salazar's report hinted at a post-Columbine cover up when it quoted former detective John Hicks as telling investigators that he was denied a promotion when he refused to lie about death threats the sheriff's office knew about before the massacre. The reaction of Hicks' supervisor to this claim was not made available.

The report left victims' families frustrated.

"This raises more questions than it answers," said Dawn Anna, whose daughter, Lauren Townsend, died at Columbine. "I would disagree that there was no negligence."

Brooks Brown said he wanted to know what sheriff's officials did with multiple warnings from his parents about Harris. He was stunned by the failure to execute the search warrant at the Harris home a year before the slayings.

"That's basically telling me my friends died because of a clerical error," he said.

His mother, Judy Brown, said she thought that only subpoenas could compel police to tell the whole story of their investigation.

"I believe Salazar will get to the bottom of the truth," she told CBS. "And I think there will be truth that there was a cover-up."