The Mexican government has said a gas leak caused a blast that killed at least 37 people at the offices of state oil monopoly Pemex in Mexico City.
Attorney General Jesus Murillo said no trace of explosives was found at the site of the explosion, the latest in a string of disasters to hit the oil giant.
President Enrique Pena Nieto is seeking to overhaul Pemex as part of a raft of economic reforms aimed at boosting growth Mexico.
"We have been able to determine that the explosion was caused by an accumulation of gas in the basement of the building," Mr Murillo told a news conference in Mexico City.
He said the gas was believed to be methane.
Mr Murillo said the gas may have leaked from containers in a storage facility connected to where the explosion took place by a tunnel. Or it could have leaked from an aging pipeline that passed through the building.
Another possibility is that it emanated from sewage in the ground under the building, he said.
Mr Murillo said contractors working on supports under the building needed electricity and used an extension cord, which could have caused a spark that ignited the gas.
Thursday afternoon's blast at a building at the Pemex headquarters complex in downtown Mexico City prompted speculation the incident could have been an act of sabotage.
That raised fears that drug war violence that has killed an estimated 70,000 people in the past six years could have entered a new, more sinister phase, and rattled investors.