NEWBURYPORT, Mass. – An explosion at a construction site that left a father and son severely burned was an accident, according to the state fire marshal’s preliminary report.

The workers were applying a flammable waterproof coating to a small basement area in a house under construction on Old Point Road at about 10 a.m. Saturday.

The pilot light in a nearby water heater ignited the vapors from the coating and sparked the explosion, Fire Marshal Stephen D. Coan said.

“It’s just a terrible accident that occurred,” Coan said.

Four workers were in the house when the explosion occurred, according to Newburyport Fire Lt. Rick Page. Neighbors dragged the injured workers form the building.

A father and son from Maine were taken to Massachusetts General Hospital in Boston for treatment of severe burns.

Their names and conditions have not been released. The two other workers were treated for minor injuries.