BRISTOL, Pa.—A lightning strike is to blame for the storage tank fire at a Philadelphia-area chemical plant that prompted nearby schools to shut down over the strong odor from chemicals released from the burning tanks, officials said Wednesday.

Lightning from storms that moved through the area overnight was blamed for the early morning blaze at the Rohm and Haas plant in Bristol. Flames engulfed a pair of 250,000-pound storage tanks as plant emergency workers and firefighters from nearby departments battled the three-alarm fire.

Plant owner Dow Chemical said the chemicals used in the production of paints were contained by a dike surrounding the tanks and no health impacts were anticipated.

Plant employees were evacuated because of the strong odor associated with the released chemicals, Dow officials said.

Emergency officials recommended residents close their doors and windows to limit exposure to the odor and the Bristol Borough School District canceled classes for the day.

The fire was officially declared under control around 7:30 a.m., four hours after it started.

A fire police officer who had been directing traffic at the scene died of a heart attack after going home because he felt ill, fire officials said.