A report released Thursday on the Columbine High School massacre reveals that police had at least 15 contacts with the two teenage gunmen before their deadly 1999 shooting spree.
Officials also released new videotapes Thursday that showed Eric Harris (search) and Dylan Klebold (search) walking in the black trench coats they wore on the day they killed 13 people before turning their guns on themselves. The tapes show Harris and Klebold going through the school and firing mock guns, perhaps at students acting as bullies for a school project.
Briefing families of Columbine victims before releasing the report to the public, Colorado Attorney General Ken Salazar (search) said authorities began having contact with Harris and Klebold up to two years before the attack.
While Salazar did not point fingers at the Jefferson County Sheriff's Office for missing warning signs that the two teens would take such a violent course of action on April 20, 1999, he said his office would investigate whether there was a cover-up.
Salazar said his staff interviewed two former sheriff's officials just this week. He also said authorities are still trying to find a file detailing a search warrant affidavit for Harris' home after a pipe bomb was found along a bike path in 1998 — a search that never took place.
"We are still looking for that file," Salazar said.
Asked if he thought there was a cover-up, he said: "I do not know today." Speaking in a room filled with somber families of the dead, Salazar promised to issue a supplemental report.
For the father of one of the victims, the report was a sad reminder of the killings. Darrell Scott, whose daughter was killed and whose son was injured, said he wished that authorities had taken action sooner against the teens.
"For that many offenses and that many reports that were called in, there should have been something done," Scott told Fox News.
But Scott did not blame authorities for the assault on the school. "Hindsight is perfect 20/20 and there's a lot of kids that joke around and say things they shouldn't say but don't go out and commit horrible acts."
Harris, 18, and Klebold, 17, killed 12 students and a teacher before taking their own lives at the school near Littleton. It remains the deadliest school shooting in U.S. history.
"In the end, none of the many efforts to open up the Columbine records, including today's activity, will mean much beyond passing curiosity if we cannot learn from this tragedy," Salazar said at a news conference.
Salazar said his investigators looked at how sheriff's officials 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 have said an anonymous tip that year led a deputy to a Web site run by Harris that said the two teens had built pipe bombs and concluded: "Now our only problem is to find the place that will be 'ground zero.'"
Family members said Salazar's report failed to give them the answers they crave.
"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."
Nearby, more than 150 people lined up to view a vast and chilling display of evidence — the murder weapons, bullet fragments, the chairs and tables where the victims were gunned down.
A message board put up in a school window that day still says in blue Magic Marker: "1 bleeding to death."
Authorities also released two videos, one of the anxious scene in a park across the street from the school that day, and another, 90-minute compilation of videos made by Harris and Klebold.
Much of the material is headed for the state archives. Relatives of the dead and survivors of the horrific attack saw much of it for the first time in a private viewing Wednesday.
"When you read about the number of bullets that were shot and you read about the number of guns, it's one thing," Darrell Scott, whose daughter Rachel was killed, said on NBC's "Today." "But when you walk into a room and see the overwhelming numbers of spent shells and bullets and pipe bombs and knives, it was just an overwhelming sight."
He said it was "the first time my wife and I had seen the gun that actually killed Rachel."
A key part of Salazar's investigation looked at work done by former sheriff's deputy John Hicks. Sheriff Ted Mink asked Salazar to investigate why a 1997 report by Hicks — found in a folder last October — was never reviewed as part of the probe into the shootings.
Hicks also looked into a 1998 complaint that Harris posted a death threat against a fellow student on the Web site, along with descriptions of pipe bombs he and Klebold built.
Randy and Judy Brown, whose son was named in the threat, reported the information to the sheriff's office. A warrant was drafted to search Harris' home, but it was never executed.
Hicks left the department in 2000 and now lives in South Carolina.
Tom Mauser, whose son, Daniel, was killed at the school, said he wanted more details about why the search never took place.
"If we're going to learn lessons, that's a key part of it," he said. "Why did law enforcement stop where it did?"
Fox News' Carol McKinley and The Associated Press contributed to this report.