Woods would not say how the company learned of the leak, but said that the Harris County Medical Examiner pronounced four of the workers dead at the scene.
The chemical, used to make insecticides and fungicides, is liquid when it is contained, but becomes a gas when it is released and mixes with oxygen, Woods said.
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"This is a tragic loss for the entire Du Pont family," Woods said. "Our thoughts are with the family members as they continue to process the loss of their loved ones."
The company would not release the names, ages or genders of the victims, at the request of their families, Woods said.
The company did not yet know how the chemical began to leak, he said.
"Our focus at the moment is solely on the employees and the family members that are impacted and the individual that is in the hospital," Woods said. "But as we move past that we will begin a full investigation to understand what went wrong and what we need to do to prevent this from ever happening again."