The government, meanwhile, announced that the death toll had risen to 33 from 32 while the Red Cross said the search for survivors had ended almost 24 hours after the explosion rocked the Mexico City complex.
"It appears that this is what one can observe as part of what experts refer to as an accident. However, no line of investigation will be ruled out," Pemex director general Emilio Lozoya Austin told the Televisa network.
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Earlier, Lozoya had refused to speculate on the cause of the blast when asked during a news conference whether the explosion was a deliberate attack.
Mexican Red Cross coordinator Isaac Oxenhaut said the search for survivors or bodies was over.
"We did a sweep with other organizations, we brought dogs again," he told reporters. "We rule out there being any trapped victims."
Soldiers continued to clear mounds of debris from the area.
