HOUSTON -- One person died and six were hurt after a small explosion and ammonia leak at the Goodyear Houston Chemical plant Wednesday morning. The body of the worker, whose name wasn’t released, was found under debris about seven hours after the morning blast.

Houston Fire Department Assistant Chief Jack Williams said Wednesday afternoon the employee who was killed was previously unaccounted for.

The gruesome discovery was a surprise to the plant manager and a surprise to the workers. At first officials thought everyone got out safely. In fact, the woman whose body was found had been accounted for.

"That's a gap in our accounting system and that will be part of our investigation,” said Mike Lockwood, the Goodyear plant manager.

“We both felt like everyone had been accounted for and sometimes if you don't have someone coming up asking where someone is that's a good sign we don't have someone missing,” said Williams.

The plant, located at 2000 Goodyear and Highway 225 in southeast Houston, makes synthetic rubber. Officials said a heat exchanger exploded around 7:20 a.m., and ammonia -- used as refrigerant to cool it down-- leaked.

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A body was found in the debris seven hours after the explosion.

Several ambulances and fire crews responded to the scene, and six people who were overcome by ammonia gas were transported to local hospitals.

Of the six initially transported, one was in critical condition and another in fair condition at Memorial Hermann in the Texas Medical Center. Two others were treated at the southeast Memorial Hermann and were released. Conditions of the two taken to LBJ Hospital were not known Wednesday night.

Fire investigators and the plant manager said it's too soon to tell if the woman who was killed would be alive had they found her sooner. They'll leave up to a medical examiner to make that determination.

And as they continue to investigate what caused the fire, though, they will try to learn more about what went wrong and why did it take more than seven hours to realize a woman was missing buried in the rubble.

There are 200 to 225 employees at the plant, and the public was not in danger, said emergency management officials.