Stratford firefighters put out multiple fires at the Army Engine Plant on Sunday.
Fire officials said it happened just before 11 p.m.
They said they battled the fires on the exterior of the building, the roof and underground-level electrical equipment.
Several units were called to assist, including the Milford and Bridgeport fire departments.
The state Department of Energy and Environmental Protection was also called to investigate.
There were no injuries reported.