A 12-year-old boy riding a push scooter set fire to the arts-and-crafts section of a Wal-Mart store in the Southern California desert, leading to losses of more than $US1 million ($A1.3 million), police say.
The store in Barstow remained closed for a third day on Friday because of the damages.
Barstow police said in a statement the boy, who was not with his parents, set the fire with a lighter on Wednesday afternoon. Wal-Mart employees had tried in vain to put it out with extinguishers, and some had to be treated for smoke inhalation.
Firefighters had put the fire out, but serious damage was done by flames and water.
Police saw the boy on surveillance video footage with a scooter and a backpack and found him on a street corner later in the day. He was arrested and taken to juvenile hall.
