A boiler explosion at the Boulder County Justice Center on Wednesday was caused by contractors installing a new heating system in the basement of the building, officials said.

Boulder police and sheriff’s deputies scrambled to the sub-basement — two floors directly beneath the probation department in the north end of the complex — after an explosion shook the building at 2:19 p.m.

Sheriff’s Cmdr. Phil West said the building was evacuated for about 15 minutes while crews determined what caused the blast and whether it was safe to be inside.

West said an aging water heater was being worked on by private contractors who narrowly escaped injury.

There was minimal damage to the maintenance room, West said, because the force of the explosion blew up a ventilation pipe to the top of the building.