Mesa Fire Department spokesman Forrest Smith says Tuesday's fire at the 1.3 million-square foot plant was contained to the roof and surrounding area of loading docks; there is no fire inside the building.
Firefighters quickly went on the attack and had the blaze under control in about 30 minutes. The loading docks are an open area, but they suffered structural damage.
Smith says 12 people had to be evacuated, but no one was injured. He says the intense blaze may have been sparked by rooftop solar panels.
The cause of the fire is under investigation.
Apple bought the plant in 2013 to manufacture sapphire glass for its products. That effort failed, and the company announced in February that it would convert the plant into a data center.
Copyright 2015 The Associated Press modified.