Investigators from the Chemical Safety & Hazard Investigation Board (CSB) are in Texas probing an apparent chemical leak that killed four workers and injured a fifth at a DuPont plant in La Porte, Texas.

The workers probably died from exposure to methyl mercaptan while responding to a valve leak around 4 AM on Nov. 15, DuPont said in a statement. The community around the plant was not at risk, the company adds.

“Our goal in investigating this accident is to determine the root cause and make recommendations to prevent any similar accidents throughout the industry,” CSB Chairman Rafael Moure-Eraso says.

Methyl mercaptan is a colorless gas that can be easily ignited, according to the Agency for Toxic Substances & Disease Registry. It is toxic and can cause headaches, dizziness, nausea, vomiting, coma, or death.

The DuPont facility, located east of Houston, uses methyl mercaptan to manufacture insecticides and fungicides, according to CSB. However, the chemical is more widely known as the additive in natural gas that gives it a distinctive rotten cabbage smell.

Methyl mercaptan was responsible for a deadly 2001 accident at an Atofina plant in Michigan. In that incident, workers died when the chemical ignited while a railcar was being unloaded, causing an explosion. The accident killed three, injured several others, and resulted in the evacuation of 2,000 local residents.

Dupont’s La Porte facility appears to have been fined previously for safety violations by the Texas Commission on Environmental Quality, according to reports from the state’s news organizations.

CSB has conducted four previous investigations into DuPont facilities, all in 2010. One of those accidents killed a worker in Belle, W.Va., and in another, a worker died in Buffalo.

In the statement, DuPont officials say they are working with local, state, and federal officials as they investigate the incident. The company says it is conducting its own “top-to-bottom review.”

This article is reproduced with permission from Chemical & Engineering News (© American Chemical Society). The article was first published on November 17, 2014.