The explosion -- which shook buildings as far as a mile away-- sent debris flying.
Flying debris killed a man who was walking in the parking lot of the oasis. The victim has been identified as a truck driver from northwest Indiana- 63-year-old Ronald Greenfield was from Chesterton, Indiana.
"He was passing through. We're not sure what he was doing at the time. We know he was out of his vehicle," said Sandra Rogers, Boone Co. Sheriff's Dept.
Only one worker was inside at the time. And remarkably, he escaped unharmed when the blast ripped through the building. The plant is highly automated, according to the mayor, and employs about 20 people. The rest of the 20 workers were in a nearby office building.
"It's extremely hard to believe that people walked out of that facility and not only that facility but the neighboring facility," said Mayor Fred Brereton.
It is not clear yet if it was a natural gas line that ruptured or pressure that built up in one of the four 75-ton vessels used for growing synthetic quartz crystal.
There was one other person injured, a woman who was blown by the blast, but she was treated and released at the scene. NDK will be out of commission for a while. But this building can be repaired and reused. In the meantime, OSHA will be investigating.
The blast was so forceful, many described it as a small earthquake.
"All of a sudden it just? there was this huge explosion. It sounded like- we told everybody, a bomb like terrorists. And that's the first thought, nowadays, and people think it is a terrorist attack," said Brandi Ciszek, witness.
"Very loud and the house shook like off -- felt like off the foundation," said Ruth Saldana, witness.
The explosion briefly closed the ramp from I-90 Jane Adams Tollway west to the Belvidere Oasis.
Belvidere is just outside Rockford.