A gunman took six girls hostage at the high school in this mountain town Wednesday, using them as human shields for hours before he shot and fatally wounded a girl and then killed himself as a SWAT team moved in, authorities said.

The confrontation at Platte Canyon High School unfolded just a short drive away from Columbine, the site of one of the nation's deadliest school shootings. The gunman, believed to be between 30 and 50 years old, was cornered with the girls in a second-floor classroom, and he released four of them, one by one.

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 one of the girls and then himself.

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 were investigating whether any of the girls were sexually assaulted, said Lance Clem, spokesman for the state Department of Public Safety.

"That's one of the things that we're trying to look into right now," Clem said.

According to some published reports, the gunman sexually abused some of the girls before releasing the hostages.

The wounded girl — identified by acquaintances and a co-worker as 16-year-old Emily Keyes — was taken to a Denver hospital in critical condition, but was declared dead, a hospital spokeswoman said.

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."

Keyes was a member of the volleyball and debate teams and was getting involved with cheerleading, said senior Desaray Trujillo, 17, who had known her since the second grade. Keyes also worked as a waitress at a restaurant in town.

"I'm feeling a lot of things right now, I'm feeling rage, I'm mourning," said her boss, Chip Thomas. "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 sweatshirt 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.

The Associated Press contributed to this report.