"Unfortunately, this tragedy is not an isolated incident, but one in a series of incidents at this site," said Labor Secretary Alexis M. Herman. "Three workers lost their lives in explosions at this plant in less than a year's time, and 23 others were killed in a major explosion in 1989."

The March explosion took place when a runaway chemical reaction occurred in a tank containing an unknown quantity of butadiene that burst the 12,000-gallon vessel. The butadiene tank was out of service for cleaning and had no pressure or temperature gauges that could have alerted workers in the control room to the impending hazard.

Plus, workers had not been trained in safety procedures for handling butadiene, and were unaware of the potential for explosion. And while the vessel was not in use, butadiene continued to flow into the tank through a non-functioning valve that had not been properly locked out.

The plant, a Phillips Petroleum Company site doing business as Phillips Chemical Company, has been succeeded by Chevron Phillips Chemical Company, LP.