Arson has been ruled the cause of a Wednesday morning fire that started in a storage shed that was being used as a sleeping room.

The fire spread quickly to a nearby duplex and caused about $60,000 damage before the blaze could be extinguished by fire-fighters from West Valley City and Salt Lake County.West Valley Fire Marshal John Blundell said there is no question the fire was an act of arson. A flammable liquid was likely used to start the fire, he said. Three male juveniles who were seen in the area at the time of the fire are being sought for questioning.

No one was home at the time of the fire. One West Valley firefighter was treated at the scene for a smoke inhalation injury.

Blundell said the shed had been furnished and was being used as living quarters for a 17-year-old male who sometimes slept inside the duplex but is not related to the occupants there. "He had been sleeping there periodically. It had a bed and a small couch and chair with a lamp that was operated from an extension cord from the patio."

The youth was at work when the fire started, Blundell said.

A neighbor reported the fire shortly after 11 a.m. She told dispatchers the fire was spreading so quickly that it moved from the shed to the house while she was on the phone. Blundell said the fire entered the house after bursting second story windows. Both sides of the duplex were extensively damaged.