Two employees killed when hydrogen sulfide leaked at a Houston-area oil refinery were employees of a subcontractor performing maintenance work, the director of Pemex, Mexico’s state-owned oil company that operates the plant, said Friday.

The two “were in the zone directly affected, and who received the direct impact of the gas,” Pemex Director Victor Rodriguez said during a news briefing in Mexico City. Both bodies have been recovered.

Harris County Sheriff Ed Gonzalez said the two workers were killed and nearly three dozen others were either transported to hospitals or treated at the scene following the leak of hydrogen sulfide Thursday at the facility in Deer Park.