A gas buildup ignited by an electrical spark or other heat source caused the blast that killed 37 people and wounded dozens of others last week at the state oil company's headquarters, Mexico's attorney general said.

But Attorney-General Jesus Murillo Karam said investigators were still looking for the source of the gas, and revising records of building inspections to determine why Petroleos Mexicanos had not discovered the gas accumulation. As a state company, Pemex is responsible for inspecting its own buildings.

Murillo said late Monday that an investigation by Mexican, Spanish, U.S. and British experts into the petroleum giant's worst disaster in more than a decade found no evidence of explosives in the Thursday afternoon blast that collapsed several lower floors of the Pemex administrative building.

He said the investigators believe that an electrical spark or other source of heat had detonated the gas.

With the exception of three victims, none of those killed had the burn marks or damaged ear drums that are typical evidence of a bombing, he said. Nor was there any sign of a crater or fracturing of the building's steel beams, also common signs of the detonation of an explosive device.

Murillo said officials had yet to discover the source of what initial evidence indicated to be methane gas that leaked from a duct or tunnel or came from the sewer system and built up in the basement of the building.

Murillo said that an independent contractor had told investigators that he was working with a crew of three men performing maintenance in the basement of building B2. The contractor said the basement wasn't lit, so his crew had rigged illumination by attaching a crude electric cable to a power source in the ceiling.

The contractor told investigators that seconds after he moved to a higher floor, he heard a noise and then the building was rocked by an explosion. The three men were found dead in the lower basement with burn marks, one with a fragment of cable stuck to his body. They had no evidence of the dismemberment typical in the detonation of explosives.

Murillo described the blast as a "diffuse" explosion whose blast moved slowly and horizontally, 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.

"We've been able to determine that the explosion was caused by an accumulation of gas in the basement of the building," he said. "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."

The announcement late Monday ended days of a near-total lack of information about the potential cause of the incident. The sparse information spawned a torrent of complaints about government secrecy and speculation about the cause of the blast, most focusing on the possibility that it was intentional.

The suspicions of foul play became so intense that Murillo insisted on displaying photos of a backpack found in the rubble in order to prove to the public that it contained makeup, and not a suspicious, potentially explosive device as reported by some Mexican media earlier in the day.

Some observers unfavorably compared the lack of openness by the Institutional Revolutionary Party government to the secretiveness of the party during its decades of autocratic rule of Mexico. The party, known by its Spanish initials PRI, returned to power in December after losing the Mexican presidency 12 years earlier.

The blast also generated 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 is not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.

The 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.

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