Officials also announced one of the patients from the UNC Burn Center died, bringing the death toll to four people. The deceased was identified as 22-year-old Kevin Cruiess.
The theory developed by the U.S. Chemical Safety and Hazard Investigation Board on Thursday is that dust generated in the rubber-making process may have contributed to the fire.
Officials from other agencies, including the Bureau of Alcohol, Tobacco, Firearms and Explosives, the lead agency in the investigation, would not speculate on a possible cause for the destruction at the West Pharmaceuticals Services plant. They would like to possibly enter the facility over the weekend.
A corner of the multistory plant continued to burn Friday as firefighters doused it with water.
Investigators said Friday that dust explosions are not uncommon, but that dust alone couldn't have led to the enormous blast.
"Dust by itself doesn't have any spark in it. We're looking for a source of ignition," said Charles Jeffress, a spokesman for the chemical safety board. "When dust gets suspended in the air some dust is more explosive than others."
He noted the plant has made rubber safely for years.
"We can't speculate at all," Jeffress said.
Don Morel, the president and chief executive officer of West Pharmaceuticals, said the plant used a dust suppression system that was triggered by a concentration of particles in the air.
"As far as we know it was working," he said.
Fuel sources for the plant also appeared secure, Morel said. The natural gas line and tanks for propane and diesel fuel outside of the plant appeared intact.
The blast shot flames and debris high into the air and shook buildings miles away. About 130 people were inside the plant, and 14 remained hospitalized at the UNC Burn Center Friday, 9 of them in critical condition.
Three workers are still in Lenoir Memorial Hospital and one is at Pitt Memorial Hospital.
The process of making the rubber, known as polyisoprene, produces "significant quantities of dust," the board said in a news release.
The building was stripped to its steel girders in the explosion and fire, and emergency officials were concerned about the structure's stability as well as whether the site was chemically contaminated.
"We are worried at this time that it will collapse," said Chief Deral Raynor, the fire official in charge of the scene.
Investigators were waiting Friday for cranes to lift the steel beams that, along with debris, hindered firefighters' ability to extinguish the fire and let investigators in. Morel said he expected his workers to go in alongside investigators to help point out what was in the plant.
Investigators also planned to interview at least 35 plant workers and others; they interviewed 140 people Thursday.
Chemical board member Dr. Andrea Taylor estimated damage to the building at $150 million.
Though some officials dismissed the idea that the blast was due to a criminal act, ATF agent Earl Woodham said his investigators will research that possibility until they rule it out.
"Any time you have a fatality and explosion combined, the ATF gets very interested," Woodham said.
The three dead workers were identified late Thursday as Faye Wilkins, 50; William Gray, 51, of Snow Hill; and James Byrd, 60, of Dover, by the funeral homes handling arrangements for memorial services.
A spokesman for the Kinston Chamber of Commerce says some of the workers have already been offered jobs at the local Pepsi Plant.
Aside from Pepsi, U.S. Cingular has offered free cell phone service to West employees during the rebuilding. A couple of local restaurants, especially King's Restaurant and McCall's Barbecue have been supplying food.
The first rescue workers at the scene said they were amazed more people weren't killed.
"I was looking for 30 to 40 people to be dead, said Tommy Howard, chief of a local volunteer fire department.
Plant worker Wayne Brown said relatively few people were needed to run the rubber-making machinery in the factory's 40-foot-tall mixing section.
Brown, who was not at the site when the blast happened, said two mixing machines feed molten rubber into two mills that create rubber sheets. Each machine requires only one person to operate it.
Workers in the plant's other section cut the sheets into pieces that are used in various medical equipment.
The state Division of Occupational Safety and Health said the plant was inspected in October, cited for safety violations and fined about $10,000, which was reduced to about $9,000 early this month.
North Carolina Deputy Labor Commissioner John Johnson said the violations were relatively routine.
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