Four workers employed by DuPont were killed and one was injured in the early morning hours of Nov.15 during a leak methyl mercaptan at a production facility of the US chemical giant at La Porte, Texas, east of Houston.

The plant uses the chemical as a starting material for crop protection products. DuPont said the cause of the accident had not been determined. The U.S. Chemical Safety Board is investigating

Facility management said the employees had responded to the leak in a valve.

Some 320 of the employees of the plant work for DuPont. Four other companies are also tenants at the complex.

The five workers were the only ones in the unit at the time of the incident and were exposed to the chemical, DuPont said. Four died at the plant, and one was still hospitalized on Nov. 17.