BAILEY, Colo. — A gunman took six girls hostage at the high school in this mountain town yesterday, holding authorities at bay for hours before shooting one of the girls, then killing himself as SWAT team members moved in. The girl later died of her injuries, a hospital spokeswoman said.

The gunman, believed to be between 30 and 50 years old, was cornered with the hostages in a second-floor classroom and released four of them, one by one. Park County Sheriff Fred Wegener said authorities decided to enter the school to save the two remaining hostages after the suspect cut off negotiations and set a deadline. He said the suspect had threatened the girls throughout the four-hour ordeal and had shielded himself with the hostages. The gunman was not immediately identified, and the sheriff was at a loss to explain a motive.

“I don’t know why he wanted to do this,” Mr. Wegener said, his voice breaking. After the suspect entered the building, hundreds of students at Platte Canyon High School were evacuated in a scene that recalled the horror at Columbine just a short drive away.

Students said the bearded suspect wore a dark blue hooded sweatshirt and a camouflage backpack. The sheriff said the man threatened to set off a bomb that he claimed to have in the backpack. The man was also toting a handgun.

Authorities had what they described as “sporadic” negotiations with the suspect and urged him to contact them for more discussion. Officers eventually crept close to the building, and some reported an explosion inside. A short time later, someone wheeled a gurney inside and took an unidentified person to a medical helicopter parked on the school’s football field. Lynn Bigham, who said she was a family friend of the murdered girl, said she had just turned 16.

“She’s real bubbly. Every time you see her she gives you a hug,” she said.