Mexico's attorney-general says a buildup of gas in the basement of the headquarters of the national oil company caused a blast that killed 37 people and wounded dozens.
Jesus Murillo Karam says an investigation by Mexican, Spanish, US and British experts found no evidence of explosives in the blast that collapsed several lower floors of the Petroleos Mexicanos administrative building on Thursday afternoon.
He said Monday that the experts believe that an electrical fault had caused a spark that detonated the leaking gas. There was scant evidence of the burn marks typical in a bomb blast, he said.
There was also no sign of a crater like that typically left by an explosive device.