The Mexican government said on Monday that a gas leak caused a blast that killed at least 37 people at the offices of state oil monopoly Pemex in Mexico City, raising fresh questions about the firm's safety record.
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 lumbering oil giant.
New President Enrique Pena Nieto is seeking to overhaul Pemex as part of a raft of economic reforms aimed at boosting growth in Latin America's No. 2 economy.
"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.
Mexico City is built on a dried-out lake bed, and the stench of sewage often hangs over parts of the downtown.
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.
The explosion next to Pemex's flagship tower block prompted renewed criticism of the oil giant's safety record. While the company had said it improved safety prior to the blast, fires, explosions and other safety breaches are regular occurrences.
There were mixed responses on the streets of the Mexican capital to the government's news conference about the blast.
Fernando Chapa, 61, a university administrator, said the evidence seemed credible and that it looked like an accident.
"It doesn't suit anyone having an attack there. These are old buildings that have leaks. It's like in the mines," he said.
But supermarket worker Jorge Lopez was not convinced.
"I don't believe it. What a coincidence that there was a gas buildup," said Mr. Lopez, 28. "These are the results they give us after all these days? I don't know..."
For years a source of national pride, Pemex has proven stubbornly resistant to change. The firm has become a touchstone for Mexico's capacity for economic reform since oil output began to fall behind the performance of other major producers.
A symbol of Mexican self-sufficiency since President Lazaro Cardenas expropriated U.S. and British oil companies in 1938 and nationalized the oil industry, Pemex has also become a byword for inefficiency and graft.