Two workers were injured at the time of the explosion.
Officials received a call of a fire around 4:30 a.m. at 111 W. High St.
The Defiance and Rescue Division arrived on scene to fight to contain flames as other fire departments showed up.
Traffic was blocked from entering Clinton Street near downtown Defiance. The roads had been shutdown for several hours and reopened around 7:30 a.m.
Officials said the building sustained minimal damage.
The Defiance County Sheriff, the Defiance Police Departments, Noble and Highland Township Fire, and the State Fire Marshal's office are currently investigating as to what caused the explosion.