At least two people are dead, and nearly three dozen others suffered injuries after a chemical leak at an oil refinery Thursday, prompting a shelter-in-place order for two East Texas cities, officials said.

The leak took place at Pemex Deer Park, about 18 miles east of Houston, the Harris County Sheriff's Office said.

The department reported deputies, and the Deer Park Fire Department responded to the facility at 5:23 p.m. local time for a report of a hydrogen sulfide leak, a colorless gas with a strong odor.

The facility where the incident occurred was a refinery that processes crude oil to produce gasoline and other fuels.

Officials announced a shelter-in-place order for Deer Park and Pasadena after the leak.

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Pemex chemical leak leaves 2 dead, 35 treated for injuries

In its initial alert Thursday, officials reported several people had suffered injuries, and one was killed in a chemical incident.

In a late Thursday night update, the sheriff's office announced two people had died, and 35 people had received medical treatment in connection to the leak. The agency also said the shelter-in-place order had been lifted.

Sheriff's office Senior Deputy Thomas M Gilliland told USA TODAY Friday that five of the injured were transported to local hospitals to be treated for unspecified injuries.

PEMEX Deer Park said a "gas release was reported at one of its operating units" at 4:40 p.m., ABC News reported.

The cause of the leak remained under investigation Friday and the extent of the injuries people suffered were not immediately released by officials.

USA TODAY has reached out to Pemex and Deer Park Mayor Jerry Mouton.

What is Hydrogen sulfide?

Hydrogen sulfide is a highly toxic and flammable gas that contains a foul odor best resembling the smell of rotten eggs, according to the federal Occupational Safety and Health Administration.

Produced naturally by bacterial decomposition of organic matter, it's found in sewage, natural gas, and volcanic gases. 

The gas is typically used in oil and gas refining, mining, tanning and paper processing. Its presence makes work in confined spaces potentially dangerous partly due to it being heavier than air, the agency warns.

Chemical leak remains under investigation

The incident remained under investigation by the sheriff's office Friday morning, the agency said.

Earlier on Thursday, officials lifted the shelter-in-place at 9:30 p.m. after receiving air monitoring reports from Harris County Pollution Control, Harris County Fire Marshal's Office Hazardous Materials Response Team, and CTEH, the sheriff's office said

"We are aware of the odor but there is no hazard to the community," the City of Deer Park posted on social media. "Thank you for your patience."

This story has been updated to add new information.

Natalie Neysa Alund is a senior reporter for USA TODAY. Reach her at [email protected] and follow her on X @nataliealund.