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 Monday.

Attorney general Jesus Murillo Karam said 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 they believed 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.

Murillo said officials had yet to discover the source of the gas, which is believed to have been methane that leaked from a number of ducts and tunnels that passed under or connected to the building, built up from the sewer system.

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.