Federal investigators are revealing the latest on their investigation into the four deaths at a DuPont pesticides plant in La-Porte last month.
The U.S. Chemical Safety Board says that there were problems with the system that handled a toxic chemical, methyl mercaptan.
Investigators said chemical fumes flowed into a building that had ventilation fans that were not operating.
The safety board said workers were not wearing air respirators as they tried to fix the malfunctioning equipment.