The white powder that workers at the U.S. Post Office in Ranson saw was most likely smoke from the damaged bulb, said Ronald Fletcher, a firefighter with Citizens Fire Co. and the designated incident spokesman.
X-rays also revealed that a suspicious package in a storage locker contained only cookies, he said.
The scare Friday morning triggered an evacuation of the building and the temporary quarantine of 15 people inside a school bus. Hazardous materials and bomb experts, State Police units, fire crews and more responded to the scene at a strip mall.
The Berkeley County Sheriff's Department sent a robot into the building to test the air. It found no chemical agents or evidence of an explosion.
Despite the six-hour disruption, postal authorities said the mail would still be delivered Friday.
Postmaster Steve Parrill said about 20 people work in the Ranson branch.