Oshawa firefighters responded to the fire on Bloor Street West around 2:50 a.m. on June 13.
The blaze destroyed the building, along with 14 motor vehicles, causing an estimated $6 million in property damage.
There were no injuries but several firefighters and police officers suffered smoke inhalation and were taken to local hospital for treatment.
The Office of the Ontario Fire Marshal was called in to investigate the fire, and has deemed arson as the cause.
The investigation remains ongoing, but police have no suspect descriptions at this time.
Anyone with information is asked to contact D/Cst. Powell of the Central East Division Criminal Investigations Bureau at 1-888-579-1520 ext. 2739.Anonymous information can be reported to Durham Regional Crime Stoppers at 1-800-222-8477 or online at durhamregionalcrimestoppers.ca and tipsters may be available for a $2,000 cash reward.