A leaky valve in the methyl mercaptan production tract of DuPont's La Porte may have been at least partly to blame for the accident that killed four workers and injured a fifth on Nov. 16.
US news media quote the plant manager as saying that some of the workers were responding to the leak when they were overcome by toxic fumes.
The federal Chemical Safety Board has sent a seven-member team to Texas to investigate the accident, which officials said which killed "an unusually high number of people for an incident that didn't involve a fire or explosion."

According to the Wall Street Journal (WSJ), the focus of the probe is likely to be how the accident occurred despite a thicket of federal regulations.
Other media reports confirmed that the Texas plant was last inspected by the Occupational Safety and Health Administration (OSHA) seven years ago, when DuPont was fined $1,700 each for two serious safety violations.