Pelion High School students returned to class Friday afternoon, after being evacuated earlier in the day following the discovery of a training grenade in a box on campus, Lexington 1 officials said.

Nearly 850 students, teachers and other staff members were evacuated for about an hour midday, using a fire drill procedure that took them out to the football field.

Officials don’t know how the grenade came to be at the school and why it was in a science laboratory, Lexington County Sheriff’s spokesman Maj. John Allalrd said.

Deputies took it away.

It’s unclear if the device was explosive, Allard said. But military training manuals say the device generally is as strong as a few cherry bomb fireworks combined.

The Sheriff’s Department is investigating.