MEXICO CITY - 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 earlier this week, Mexico's attorney-general said Sunday.
Attorney-General Jesus Murillo Karam said an investigation by Mexican, Spanish, U.S. 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 they believed that an electrical fault had caused a spark that detonated the leaking gas. There was scant evidence of the burn marks or evidence of a blast wave typical in a bombing, he said. There was also no sign of a crater like that typically left by an explosive device.
Murillo said officials had yet to discover the source of the gas, which is believed to have been methane gas that either leaked from a number of ducts and tunnels that passed under or connected to the building, or built up from the sewer system.
He described the blast as a "diffuse" explosion, typical of the detonation of a cloud of gas, rather than an explosion that would have emanated from a relatively compact source like a bomb.
He said laboratory tests had turned up "zero" evidence of any explosive.
"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," he said.
The announcement late Monday ended days of silence about the potential cause of the company's worst disaster in a decade. The blast fueled debate about the state of Pemex, a vital source of government revenue that is suffering from decades of underinvestment and has been hit by a recent series of accidents that have tarnished its otherwise improving safety record.
Until now, virtually all the accidents had hit its petroleum infrastructure, not its office buildings.
Mexican President Enrique Pena Nieto has pledged to open the oil behemoth to more private and foreign investment, setting off warnings among leftists about the privatization of an enterprise seen as one of the pillars of the Mexican state. Pena Nieto has provided few details of the reform he will propose but denies any plan to privatize Pemex.
Murillo said there not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.
That disaster was a major setback to a safety record that had been improving following a series of incidents in the 1980s and 1990s, according to company figures. The number of accidents per million hours worked dropped by more than half, from 1.06 in 2005 to 0.42 in 2010. That is in line with the international average of about 0.43 per million, according to the U.K.-based International Association of Oil and Gas Producers, which does not independently verify company numbers.
But Pemex acknowledged in a report that starting in late 2011, a series of smaller blasts and fires, mainly at refineries and petrochemical plants, had "seriously impacted" its safety rate. It said the rate of injuries per million hours had risen to 0.54.
As part of the federal government, Pemex is entirely responsible for inspecting its own buildings. Murillo said investigators were looking into the records of building inspections, and why they had not discovered the accumulation of gas.