One worker was missing and a second injured in an explosion at a Bayer CropScience plant that left thousands of nearby residents being urged to stay indoors.

The explosion, which sent a fireball hundreds of feet into the air and could be felt miles away, occurred about 10:25 p.m. Thursday. The blast occurred in a section of the plant where waste products are treated before disposal, Bayer spokesman Mike Wey said.

The unit had been closed for maintenance and was restarted earlier in the week, he said.

Wey said the injured worker suffered burns and was transferred to a Pittsburgh hospital. A company emergency responder was treated at the plant infirmary for heat stress, Wey said.

The search for the missing worker would not start until the fire was extinguished. An eight-person crew normally works the unit. The other six workers were not injured, he said.

The plant is located about 10 miles west of Charleston and residents miles away from the facility reported hearing the explosion.

"I personally live about 10 miles from the plant and I felt it," said Kanawha County Manager Brent Pauley.

West Virginia State University Police Department patrolman Robert Flinn said he felt the explosion as he was sitting in his cruiser on the school's campus near the plant.

"Our back was turned, and it was like somebody shined a giant spotlight on us," he said.

Warne Ferguson lives less than a mile from the plant and the explosion shook his house.

"It scared my wife to death," said 81-year-old Ferguson. "I thought my house was falling down on top of me. That's how hard the vibration was."

As a precaution, emergency officials temporarily closed Interstate 64, U.S. 60 and state Route 25. Also, residents in at least five nearby communities were told to stay indoors. The highways were reopened and shelter-in-place orders lifted shortly after 2 a.m. Friday.

The material that burned had an odor, but Wey said "people should be confident they have not been exposed to highly dangerous chemicals."

Investigators will work to determine the cause of the explosion once it's safe to enter the unit, he said.