About 1,000 residents were evacuated from the area after the explosions Friday night, said Sheriff's Sgt. Joe Rath. As firefighters continued to battle the blaze Saturday, all but about 60 who live within a mile of the plant returned to their homes, he said.
The explosions occurred at about 10:45 p.m. in a chemical reactor at the Formosa Plastics plant, said Illinois Emergency Management Agency spokeswoman Patti Thompson. The cause of the explosions was unclear.
A witness who was driving on the interstate said he saw an orange flash similar to lightning, followed by a muffled boom, then an enormous fireball 100 feet high.
Two bodies were discovered early Saturday as small crews of firefighters began to surround the plant, which was "60 or 70 percent destroyed," said police Capt. Jeff Burkler. He did not identify the victims.
Four people were taken to Memorial Medical Center in Springfield, but the extent of their injuries was not immediately released. Four others were treated for smoke inhalation.
The dead and injured were among 18 workers who were on duty at the plant when the explosions occurred, Rath said.
Fires were still raging in several areas of the plant early Saturday and Williamson said more than 150 fire and rescue workers were expected on the scene.
A short time after the explosion, Interstate 72 was closed in both directions, from Illiopolis to Niantic, which is between Springfield and Decatur in central Illinois. The blast also knocked out power in both communities.
Thompson said the interstate was reopened shortly before 6 a.m. Saturday, after the Illinois Environmental Protection Agency had determined that smoke from the fire was not a hazard to motorists' health.
Illinois Farm Bureau spokesman John Hawkins, who said he lives about five miles from the plant, said he felt the explosion.
"We were getting ready for bed and we heard this loud boom and the house shook," he said.
The plant, with about 135 workers, manufactures polyvinyl chloride used in PVC pipe and other materials for construction, medical supplies and automotive parts.
Investigators from the U.S. Chemical Safety and Hazard Investigation Board headed to the scene Saturday to begin an investigation. Plant manager Roe Vadas said a team of investigators from the plant's corporate parent, Formosa Plastics Corp. USA, was also on its way.