What sparked the blast remains a mystery.

Investigators said the explosion began in a mixing area where small fires had occurred in the past. They also said they had identified several possible sources of dust, but could not say which was responsible.

The plant made synthetic rubber stoppers and other medical supplies, used a variety of chemicals to cure rubber. Nine workers were listed in critical condition at UNC Hospital in Chapel Hill.

Investigators from the CSB say they have identified several potential sources of explosive dust from rubber processing operations at West Pharmaceuticals and are looking into what ignited a massive explosion on the lower level of the rubber compounding area at the plant.

CSB investigators are studying whether a suspended or drop ceiling installed on the first floor above the processing area may have created an inadequately ventilated space and caused an accumulation of explosive dust.

The investigators are also examining whether the large explosion on the lower level may have been triggered by a smaller accidental explosion of unknown origin.

Plant employees told the investigators that the rubber mixer on the upper level had experienced previous internal fires, including one strong enough to blow off the mixer door.

Investigators do not yet know the number or nature of these fires or whether there is a connection between these mixer fires and the events of January 29.

Materials present at the site that could produce explosive dusts include sulfur, polymer powders, and other organic processing agents. Federally required Material Safety Data Sheets (MSDSs) issued by the suppliers of some of these materials carry warnings about the possibility of dust explosions.

The West plant processed polyisoprene and others forms of synthetic rubber into stoppers and used a variety of rubber curing, antistick, and processing agents.

Rubber was blended with vulcanizing chemicals in a mixer on the upper floor and then dropped through a trap door to a processing mill on the lower level.

In the mill, the rubber mass was rolled, coated with polymer powder, and then dried and cut.

After viewing the destroyed facility, lead investigator Steve Selk said, "Dust can be an insidious hazard wherever finely divided organic materials are used. While some of these materials might appear to present a limited fire hazard, when suspended in air under the right conditions these same materials can explode with deadly consequences. Recent fatal accidents in polymer processing facilities - including those in Vicksburg, Mississippi, and Richmond, California -- highlight the dangers involved."

See the CSB Web site at: http://www.chemsafety.gov/news/2003/n20030201.htm