HOUSTON -- Four workers killed by poisonous gas during a recent chemical leak were trapped inside the Texas pesticide plant for an hour before anyone called 911, and no one told dispatchers what substances were inside. The DuPont plant in La Porte, Texas typically housed as much as 250 tons of highly flammable methyl mercaptan, the Houston Chronicle reported Sunday. Must Read: 12 Stocks Warren Buffett Loves in 2014 But it also contained at least some methyl isocyanate.