Searchers recovered four more bodies from a petrochemical plant wrecked by a huge explosion this week on Mexico's southeastern Gulf coast, raising the death toll to 28, the state oil company said Friday night.

Petroleos Mexicanos said in a statement that some workers were still missing, but did not specify a number. It had said Thursday that eight workers were unaccounted for.

Pemex said some areas of the plant had not yet been searched. Earlier in the day, it said drones and tracker dogs were aiding the search.

The company said 18 people were still hospitalized. More than 130 people in all suffered various injuries Wednesday when the explosion rocked the plant in Coatzacoalcos, about 600 kilometers (370 miles) southeast of Mexico City.

Pemex has said the explosion happened after a leak but has not determined the source.

The Clorados 3 plant of Petroquimica Mexicana de Vinilo produces the hazardous industrial chemical vinyl chloride.