The confrontation Wednesday at Platte Canyon High School in Bailey, Colo., unfolded just a short drive away from Columbine, the site of one of the nation's deadliest school shootings.
CBS News correspondent Tom Foty reports the gunman, believed to be between 30 and 50 years old, was wearing a hooded sweat shirt and a camouflage backpack when he suddenly walked into a classroom, fired a gun, and lined up the frightened students against the blackboard.
Police say eventually he picked out six of the female students as hostages, and later let four of them go – one by one – keeping two as human shields.
Park County Sheriff Fred Wegener said authorities decided to storm the classroom after the man cut off negotiations and set a 4 p.m. deadline. Wegener wouldn't say what the man threatened to do. He said authorities used explosives as they entered the classroom, only to have the suspect fire at officers, shoot a 16-year-old girl, and then shoot himself.
The girl, whose name has not been released by police, was taken to a Denver hospital in critical condition, but doctors were unable to save her.
The man was not immediately identified - one official declared him a virtual John Doe - and the sheriff was at a loss to explain a motive.
"I don't know why he wanted to do this," Wegener said, his voice breaking.
Authorities are investigating whether any of the girls were sexually assaulted, said Lance Clem, spokesman for the state Department of Public Safety.
The last hostage was unharmed and spoke with authorities. School was canceled for the rest of the week.
"We are a community in mourning," schools superintendent Jim Walpole said. "Our thoughts, our prayers are with our students, staff and their families. Especially the family of the student we lost."
Friends say the dead student worked as a waitress in town, was a member of the volleyball and debate teams, and was getting involved with cheerleading, according to a fellow student, Desaray Trujillo, 17, who has known her since the second grade.
"I'm feeling a lot of things right now, I'm feeling rage, I'm mourning," said Chip Thomas, who worked with the teen at the restaurant. "The senselessness of it all, is where the rage comes from. She was a good kid, she'll be missed."
After the suspect entered the building, hundreds of students were evacuated. The sight of students fleeing the high school in long lines, and of frantic parents scrambling to find their children, evoked memories of the 1999 attack on Columbine High School, where two students killed 13 people before committing suicide.
Students said the bearded suspect wore a dark blue hooded sweat shirt and a camouflage backpack. The sheriff said the man claimed to have a bomb in the backpack and threatened to set it off. The man was also toting a handgun.
Tom Grigg said his 16-year-old son, Cassidy, was in a classroom when the man walked in, fired a gun and began telling some students to leave and others - all girls - to stay.
"He stood them up at the blackboard," Grigg said. "He hand-picked the ones he wanted to get out."
The gunman told Cassidy to leave, but he said he wanted to stay with the girls, Grigg said.
"The guy flipped him around and put the gun in his face and said, 'It would be in your best interest to leave,"' Grigg said.
Students described a chaotic scene inside after the intercom announced "code white" and everyone was told to stay in their classrooms.
The high school and a nearby middle school were soon evacuated. Jefferson County authorities - who also handled the attack at Columbine - sent a bomb squad and SWAT team to the high school.
"I'm just terrified. I'm terrified," said Sherry Husen, whose son plays on the high school football team and was told not to return to school from his part-time job. "I know so many kids in that school."
Students from the high school and a nearby middle school were taken to another school for a head count. Ambulances were parked in the end zone of the high school's football field, and a tank-like SWAT team vehicle was parked nearby on a closed highway.
Parents pressed authorities for details but had little information on their children.
Bill Twyford said he received a text message from his 15-year-old son, Billy, a student at the high school, at about 11:30 a.m. It said: "Hey there, there's a gun hijacking in school right now. I'm fine, bad situation though."
Michael Owens, who has one son at the middle school and another in the high school, said the anxiety was worse because of the memory of Columbine.
"Things that are out of your control," he said. "It's like an earthquake."
Tom Mauser, whose son Daniel was among the students slain at Columbine, said: "Any adult who holds kids hostage is reprehensible."
The schools are in a narrow, winding canyon carved by the South Platte River about 35 miles southwest of Denver. They have an enrollment of about 770 students, with 460 in the high school.
Husen's family moved to Bailey from suburban Denver about 14 years ago.
"We moved up here for the mountain solitude, and I just never thought this would happen in this school, but it happens everywhere," she said.