BROOMFIELD — Police arrested a student at a Broomfield middle school Thursday morning after they say he stabbed a 14-year-old girl in the back with a kitchen knife as children filed in from the playground.

Authorities say the student, a 14-year-old boy, was detained by teachers who were monitoring the recess. The girl was flown by helicopter to a hospital in stable condition.

Officers were called to Aspen Creek K-8 School at about 11:30 a.m. and began interviewing the boy, who police say is cooperating with investigators.

“It sounds like it may be a serrated kitchen knife,” Sgt. Mark Goodell, a police spokesman, said of the weapon.

A large area surrounding the playground was cordoned off in yellow police tape as officers investigated. Authorities say they don’t know where the boy got the knife or how he got it into the school.

Boulder Valley School District spokesman Briggs Gamblin said there were teachers supervising nearby when the girl was stabbed.

“This evidently happened as they were going in,” Gamblin said. “It sounds like the person being detained is being cooperative right now.”

School officials and police stressed that the stabbing appeared to be an isolated incident and that there was no reason to believe there is any additional threat to the school community.

“There are witnesses, and police are talking with them,” Gamblin said.

Parents of children who saw the stabbing have been notified, and a letter was sent home to the school community, Gamblin said. School continued as normal for the rest of the day, although parents who wanted to pick up their children early were allowed to.

“This is very, very concerning,” Gamblin said.

Additional counseling and support resources have been assigned to students and staff members at the school.

Goodell said investigators don’t know what motivated the attack.

John Aguilar: 303-954-1695, jaguilar@denverpost.com or twitter.com/abuvthefold