Sheriff Fred Wegener said the assaults went beyond touching or fondling.
"It was pretty horrific," Wegener said, without elaborating.
The killer was identified as 53-year-old Duane Morrison, a petty criminal who had a Denver address but had apparently been living in his battered yellow Jeep when he walked inside the school Wednesday with two handguns and a backpack that he claimed contained a bomb. Investigators did not immediately say what was in the backpack.
Another question is how he got into the school to begin with and was able to wander around unchallenged. Sophomore Billy Twyford was one of the first to see the grey-haired man and knew something was off about him.
"I just saw him and I was like, 'I'm staying away from him, he's weird,'" Twyford told CBS News correspondent Cynthia Bowers.
Authorities said they knew of no connection between Morrison, his hostages or anyone else at Platte Canyon High School in this mountain town of about 3,500.
During the siege, he took the girls hostage in a second-floor classroom and eventually released four of them. Morrison, still holding two girls, soon cut off contact and warned that "something would happen at 4 o'clock," authorities said.
About a half-hour before the deadline, a SWAT team used explosives to blow a hole in a classroom wall in hopes of getting a clear shot at him, but they couldn't see him through the gap, and they blew the door off the hinges to get inside, said Lance Clem, a spokesman for the state Department of Public Safety.
Morrison fired at the SWAT officers, shot 16-year-old Emily Keyes in the back of the head as she tried to run away, and then killed himself, authorities said. During the lightning-fast gun battle, police said, they shot Morrison several times.
A sorrowful Wegener defended the decision to try to take Morrison by force.
"My decision was to either wait, with the possibility of having two dead hostages, or act to try and save what I feared he would do to them," the sheriff said. "We have confirmed he did traumatize and assault our children. ... This is why I made the decision I did.
"We had to go try and save them."
Classes were canceled for the rest of the week as the community tried to come to grips with the bloodshed, which evoked memories of the 1999 shooting rampage at Columbine High School, less than an hour's drive away, that left 15 dead.
"This is — this is something that has changed my school, changed my community," the sheriff said. "My small county's gone."