"It sounded like a traffic accident -- a boom," said Greg Brown, a school district official. "We thought someone dropped something on the roof. It was like a little earthquake."
The man who was killed was struck by debris and pronounced dead at the Illinois Tollway Oasis on Interstate 90, Illinois Tollway spokeswoman Joelle McGinnis said. Belvidere police and Mayor Fred Brereton said the man was putting gas into his car at the rest area, which also has several restaurants. McGinnis could not confirm that.
State Police were trying to reach the man's family before releasing his identity.
Jim Fealtman, a regional sales manager at NDK America's office in Webster, Mass., the company's other U.S. location, said no one at the Belvidere plant was injured in the blast. He had no other information.
Three sides of the NDK plant, which makes synthetic crystals for computers, collapsed in the blast, WIFR-TV in Rockford reported.
The entrance to the plant, which is in a large empty field, was blocked Monday night. The rest area is visible from the plant.
NDK had moved to a new 55,000-square-foot factory in Belvidere in September 2003 and employs about 30 people, according to Growth Dimensions, the economic development agency for the city of Belvidere and Boone County.
The building housed four 75-ton "autoclaves" -- each 50 feet tall and 3 1/2 feet in diameter -- in which the crystals were grown.