The blast killed 37 people and injured dozens of others. It was the petroleum giant's worst disaster in more than a decade.
Attorney-General Jesus Murillo Karam said an investigation found no evidence of explosives in the blast that collapsed several lower floors of the Petroleos Mexicanos administrative building.
"We've been able to determine that the explosion was caused by an accumulation of gas in the basement of the building," he said. "This explosion, at its peak, generated an effect on the structures of the floors of the building, first pushing them up and then causing them to fall, and that was the primary cause of deaths in the building."
The methane gas was apparently ignited by a spark from an electrical fault. The source of the methane gas has not yet been identified.
With the exception of three victims, none of those killed had the burn marks or damaged ear drums that are typical evidence of a bombing, the attorney-general said. There was also no sign of a crater or fracturing of the building's steel beams, which are common signs of the detonation of an explosive device.
Murillo said there is not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.
The Associated Press contributed to this report.