"It's believed to be accidental," Contra Costa Fire Protection District spokeswoman Emily Hopkins said. She said the cause may have been electrical, but that has not yet been officially determined.
The fire was reported just after 1 a.m. at the Quiznos at 622 Contra Costa Blvd., and was brought under control about half an hour later -- but not before "pretty much" gutting the restaurant, Hopkins said.
"They were able to save the business next door," she said. "But the restaurant is closed."
Damage was estimated at $500,000.
Hopkins said investigators believe the blaze may have begun in the restaurant's attic space. Witnesses reported hearing an explosion, which may have been the result of oxygen getting into that overheated space, she said.
No one was in the restaurant at the time and no injuries were reported.