SAVANNAH, Ga. (AP) – Dust that collected in a piece of safety equipment caused a small explosion at a sugar refinery weeks before the deadly blast that killed nine workers, a federal investigator said Sunday.

Stephen Selk, 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,” Selk said.

No one was injured in the earlier explosion, said 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 as well as federal regulators.

It has just begun looking into the blast after criminal investigators determined the explosion was accidental – caused by clouds of 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, 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, 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, Behm said. Damage to the dust collector was minimal, and it was quickly repaired, he said.

“The equipment in there functioned correctly and did its job,” Behm said.

Selk said investigators haven’t yet determined whether the dust extraction equipment was working in the part of the refinery where the larger explosion began – a basement area with conveyor belts used to move sugar from three giant storage silos to a nearby packaging area.

Evidence of sugar dust accumulation was found in the basement beneath the silos, investigator Phil Durham of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said Friday.

Steve Behm, a spokesman for Imperial Sugar, said he did not know any details of the earlier dust explosion mentioned by Selk.

Investigators also have not determined what ignited the dust to cause the deadly explosion. The Chemical Safety Board is more concerned with why combustible dust was accumulating inside the refinery, Selk said.

“Finding the ignition source may be impossible,” Selk said.

The explosion killed eight workers inside the plant, and a ninth worker died at an Augusta hospital. Fourteen people with severe burns remain in critical condition, and two workers are in serious condition.

The refinery is on a 160-acre site on the Savannah River just west of Savannah. About 12 percent of the plant’s 872,000 square feet was destroyed by the explosion and fire.