Investigators were looking specifically at a suspended ceiling on the first-floor processing area of the West Pharmaceutical Services plant on the theory that rubber dust could have accumulated there.
``Dust is insidious,'' said Steve Selk, an investigator with the U.S. Chemical Safety Board. ``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.''
The blast Wednesday at the plant that makes syringes and IV fittings shot flames and debris into the air and shook buildings miles away. Three people died immediately and a fourth died later of burns.
Officials compared the dust to the combustible cloud that can form in grain silos, and said the blast could have been sparked by static electricity, a small fire or some other cause.
Materials used at the site that could have caused a buildup of dust included sulfur, polymer powders and other organic processing agents, the safety board said.
Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives left the site Monday after ruling out any criminal intent behind the blast.
About 130 people were inside the medical supply factory at the time of the explosion. Nine remained in critical condition Monday at the North Carolina Jaycee Burn Center in Chapel Hill.
