A hazardous chemical leak in a DuPont industrial plant has killed four workers and injured another. The leak occurred at around 4 a.m. Saturday at the DuPont plant in La Porte, Tex., about 20 miles from Houston. The release was contained within two hours, according to officials, but by then the five employees had been exposed. Four died at the scene and the other is hospitalized.

The leak was the chemical methyl mercaptan, used to create insecticides and fungicides. La Porte’s emergency management coordinator Jeff Suggs said that the chemical is not toxic for those living near the plant, but that it did cause a rotten egg smell. Another purpose for methyl mercaptan is to odorize natural gas for safety purposes.

The leak occurred when a valve failed on a container, but it is not known at this time why the failure occurred. Plant manager Randall Clements said that all the employees involved were well-trained and knowledgeable. They had worked at the plant from eight months to 40 years and all were doing the jobs they were trained for.

Two previous safety violations at DuPont facilities occurred in 2010 that resulted in employee deaths. A phosgene gas release in a Belle, W. Va. facility killed one worker. An explosion during welding at a plant outside Buffalo, N.Y. that same year resulted in another death, for which the Chemical Safety Board blamed the company’s failure to monitor flammable gas levels.

DuPont is cooperating with federal, state and local officials in an investigation to determine the cause of yesterday’s leak. The company is also conducting their own “top to bottom review” of the accident, and plan to share what they learn with the authorities. Friends of the surviving worker said that he was conscious and talking late Saturday.