Two brothers were among the victims of a deadly chemical leak at a Houston-area plant, authorities revealed Sunday.

Robert and Gilbert Tisnado both worked in the DuPont Chemical Refinery's crop unit, according to KRIV. Sources told the station that one of the brothers went to the aid of the other and both were overcome by the fumes early Saturday. 

The other victims were identified as Wade Baker, a first-line supervisor who had worked at the plant for 40 years, and Crystal Wise, who was the least-experienced employee, having only been hired eight months earlier. Company spokesman Aaron Woods said all the workers were at the plant carrying out their normal shifts when the leak occurred at around 4 a.m. local time. 

A fifth employee was released from a local hospital due to exposure to the fumes

DuPont officials said Sunday that it was still not clear what caused the leak of methyl mercaptan at the plant in La Porte. The chemical is used to create crop-protection products such as insecticides and fungicides.

Woods said company officials are cooperating with local, state and federal authorities in the investigation.

A team from the U.S. Chemical Safety Board, an independent federal agency that investigates industrial chemical accidents, arrived in the Houston area on Sunday to start its probe. The agency said in a news release that five of eight team members planned to receive a briefing from company officials Sunday evening, while the full team will begin work Monday.

Don Holmstrom, director of CSB's Western Regional Office in Denver, is leading the investigation team along with Investigation Supervisor Johnnie Banks. CSB Managing Director Daniel Horowitz is accompanying the team.

Methyl mercaptan also is used to odorize natural gas -- which has no odor -- for safety purposes.

The La Porte plant has 320 DuPont employees. Four other companies are also tenants at the complex.

Jeff Suggs, emergency management coordinator for La Porte, said the chemical release was not toxic for those living nearby, but that it caused a smell similar to rotten eggs.

The Associated Press contributed to this report. 

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