Jesus Murillo Karam told a news conference that investigators are still looking over records of building inspections to learn the exact origin of the gas at the Pemex compound, El Universal reported Tuesday.
Murillo said initial evidence indicates methane gas leaked from a duct or tunnel or came from the sewer system and built up in the basement of the building.
He said an investigation by a team of experts found no evidence that explosives caused the blast that collapsed several lower floors of the Pemex administrative building.
Murillo said the bodies of the 37 victims would have been dismembered had there been explosive devices.
Also, there were no traces of a fire and no crater in the building, both common signs that an explosive device was detonated, he said.