The Ontario Fire Marshal’s office is investigating a massive blaze near downtown Chatham. 
Crews were called to Church’s Army and Outdoor Military Surplus on Thames Street early Saturday afternoon.
The building also houses Grill Lite Hot Tubs.
Witnesses say there were a pair of explosions just before flames could be seen coming from the building.
It took fire fighters several hours to battle the flames, which sent toxic smoke billowing into the air.
Crews remained on site to douse hot spots into the night.
Damage has been pegged at $550,000.
The cause of the blaze remains under investigation.