A hostage situation at a Colorado high school Wednesday ended with the gunman fatally shooting a teenage girl before killing himself.

The unidentified gunman shot a female student and then himself, said Park County Sheriff Fred Wegener.

Members of a SWAT team exit the Platte Valley High School in Bailey, Colo., where an adult gunman killed a female student and then himself on Wednesday.
(Dennis Schroeder, Rocky Mountain News/Associated Press)

A spokesperson for St. Anthony Hospital confirmed the girl died, nearly one hour after being removed from the school.

Six hostages were initially taken in the afternoon at the Platte Canyon High School in the small town of Bailey, about 55 kilometres southwest of Denver.

Gunman separated male and female students

An adult gunman walked into the building, opened fire and claimed to have explosives, authorities said.

He then separated males from females and held six of the girls, witnesses told Denver's Rocky Mountain News.

A young girl was released shortly after the hostage taking, and hours later three others were allowed to leave.

Two female students remained when witnesses reported hearing an explosion around 3:30 p.m. MT, about 30 minutes before a deadline the gunman had imposed, authorities said.

It is unclear what the gunman's motive was, or whether authorities entered before or after the gunshots were fired.

Earlier, Jacki Kelley, a spokeswoman for the Jefferson County sheriff's department, had said the suspect barricaded himself in a classroom with the students.

"We have almost no information on this suspect," Kelley said. "We don’t know who he is. We don't know why he's in there. We don't know what he wants."

Kelley said a number of shots were fired in the school. She said there had been "sporadic" communication with the suspect.

Hundreds of students were evacuated from the school.

Authorities sent a bomb squad and a SWAT team to the school, said a spokeswoman for the sheriff.

Jefferson police were called in to assist the smaller Park County force.

At least four other schools in the area were locked down.

Bailey is located about 50 kilometres from Littleton, Colo., where two high school students shot and killed 13 people in Columbine High School in 1999.

With files from the Associated Press