Four workers were killed and one was injured Saturday during a hazardous chemical leak at a DuPont industrial plant in suburban Houston, company officials said.

The chemical, methyl mercaptan, began leaking from a valve around 4 a.m. in a unit at the plant in La Porte, about 20 miles east of Houston. Plant officials said the release was contained by 6 a.m.

Methyl mercaptan was used at the plant to create crop-protection products such as insecticides and fungicides, according to DuPont. The cause of the leak was not immediately known.

Five employees were in the unit at the time of the incident and were exposed to the chemical, the company said. Four died at the plant, and one was hospitalized.

"There are no words to fully express the loss we feel or the concern and sympathy we extend to the families of the employees and their co-workers," plant manager Randall Clements said in a statement. "We are in close touch with them and providing them every measure of support and assistance at this time."

The company said the fifth worker who was hospitalized was being held for observation but didn't provide further details. DuPont would only say "the employee is currently receiving treatment." None of the victims was immediately identified.

DuPont will cooperate with local, state and federal officials investigating the leak, Clements said.

"As part of that investigation, we are conducting our own top-to-bottom review of this incident and we will share what we learn with the relevant authorities," he said.

The U.S. Chemical Safety Board, a federal agency that investigates chemical accidents, announced late Saturday that it was sending a seven-person team to investigate the incident.

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 that's similar to rotten eggs.

"It's a nuisance smell in the area. It's a smell that's traveled quite far," Suggs said.

The odor from the leak lingered in the area for the better part of the day and reached areas about 40 miles away, The Houston Chronicle reported.

Methyl mercaptan is also commonly 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.