The gunman, 54-year-old Duane Morrison, took six female hostages at Platte Canyon High School in Bailey, south of Denver, after entering the building around lunchtime on Wednesday.
Four hostages were released before the bloody end to the siege, which took place after police SWAT teams decided to storm the second-floor classroom where Morrison was barricaded inside.
Park County Sheriff Fred Wegener said police were still looking at possible motives but revealed that Morrison, described as a loner who had been living out of his car, had sexually assaulted an unspecified number of his captives.
"We have confirmed that he did traumatise and assault our children," Mr Wegener told reporters. "The assaults were sexual," he added.
Local media reported that a backpack in which Morrison had claimed to have been holding a bomb contained various sex toys.
‘Very minor’ criminal history
Mr Wegener, who had been close to tears while speaking to reporters on Wednesday, said that he had taken the decision to storm the school after learning that hostages were being assaulted from captives who had been freed.
"This is why I made the decision I did," he said.
Mr Morrison, armed with a semi-automatic pistol and a revolver, fired at police before shooting his victim and turning his gun on himself, Mr Wegener said.
"The sequence was awful fast," he said. "As they burst through the door he fired at the SWAT team, shot the female hostage and then shot himself."
The schoolgirl, identified by police as 16-year-old Emily Keyes, was airlifted to St Anthony Central Hospital in Denver but died of her injuries late Wednesday.
Mr Wegener said Morrison had a "very minor" criminal history but did not say if he had any previous convictions for sexual offences.
Wednesday's shooting immediately evoked memories of the 1999 Columbine high school massacre in neighbouring Jefferson County, when 13 students were gunned down by two teenage pupils who then killed themselves.
Community shaken: sheriff
Mr Wegener, a 36-year police veteran whose son had been in the school when the drama unfolded on Wednesday, said the rural community had been shattered by the tragic events.
"This is something that has changed my school, changed my community," Mr Wegener said. "Our small community is gone."
A teenager in the classroom when Morrison entered, Cassidy Grigg, 16, told how the gunman had fired a shot at the floor and then selected his hostages, all girls.
"You could tell that he wanted the females," Cassidy said on NBC television. "He tapped me on the shoulder and he told me to leave the room.
"I told him, 'I don't want to leave.' "He told me that if I didn't go then he would pretty much kill me."