Stephen Selk, the investigations manager for the U.S. Chemical Safety Board, had few details about the previous explosion at the Imperial Sugar refinery in Port Wentworth.
He could not say whether the earlier blast contributed to the massive explosion Feb. 7.
"It is far too early to reach conclusions about the relationship between that event and this one," Mr. Selk said.
No one was injured in the earlier explosion, said Mr. Selk, who did not know the exact date. Imperial Sugar spokesman Steve Behm said it happened about three weeks ago and caused minimal damage.
The Chemical Safety Board investigates industrial accidents for the federal government and makes safety recommendations to industry and trade groups in addition to federal regulators.
It has just begun looking into the refinery blast after criminal investigators determined Friday the explosion was accidental -- caused by clouds of tiny sugar dust particles that, when airborne in confined spaces, can ignite like gunpowder.
The refinery was equipped with a network of fans and ducts designed to prevent dust explosions by sucking particles out of the plant and transferring them to dust collectors on the roof, Mr. Selk said.
It was inside one of those rooftop dust bins where an explosion occurred weeks before the Feb. 7 blast.
The earlier blast was caused by a small piece of metal that passed through a machine used to grind granulated sugar into finer particles, Mr. Behm said. The metal fragment caused a spark that got sucked into a dust collector and ignited the dust inside it, he said.
The rooftop dust collector had ventilation panels that opened to relieve pressure from the small blast, minimizing the force of the explosion, Mr. Behm said. Damage to the dust collector was minimal, and it was quickly repaired, he said.
Mr. Selk said investigators haven't determined whether the dust extraction equipment was working in the part of the refinery where the larger explosion began.