HILLSBORO, Ore. -- Six students were hurt at a Hillsboro school Friday when bleachers collapsed during class photos. Washington County sheriff's deputies said a criminal investigation is under way because supports for the bleachers had been stolen. Deputies said they weren't sure when the supports were removed but that they know they weren't taken by anyone from the school district. The Washington County Forensics Science Unit checked the bleachers for fingerprints hoping to get a lead in the case, deputies said.

A sixth-grade class of about 50 students at West Union Elementary School had gathered on the bleachers when the aluminum structure gave way at about 10 a.m.

Officials said the six students suffered only minor injuries. Three of them were immediately taken to a hospital -- one girl with a broken ankle -- and school officials left the other three students with their parents.

All of the other students involved in the accident were either uninjured or they suffered only small scrapes and bumps. The school said it's up to their parents to determine whether they will finish the day of school or head home.

The cause of the collapse is under investigation by the school facility crews and sheriff's deputies. West Union Elementary School is located on West Union Road. Firefighters and Washington County sheriff's deputies responded to the scene after the bleachers collapsed.