MEXICO CITY (AP) -- A water-heating system may have leaked gas into a tunnel beneath the headquarters of Mexico's national oil company for more than seven months before it was accidentally detonated by a maintenance crew's improvised lighting system, officials said Tuesday, adding fresh detail to the narrative of the petroleum giant's worst disaster in a decade.
Mexico's attorney general said Monday night that a gas buildup was responsible for the explosion that collapsed three floors of the administrative building in Petroleos Mexicanos' Mexico City headquarters complex, killing 37 people and leaving dozens hospitalized. He said the investigation had detected traces of methane. Methane is the primary component of most of the natural gas used for cooking and heating in homes and businesses. It also is found in coal mines, and is naturally produced by the decomposition of organic matter in sewers and landfills.
Assistant Attorney General Alfredo Castillo told reporters Tuesday morning that one source of the gas may have been a tunnel that ran from a heating plant and beneath the devastated building on its way to the high-rise central tower of the complex. He said that explanation was supported by the fact that the blast blew off manhole covers over the tunnel.
"There's a connection to, as you've seen, a place where there are gas facilities," Castillo said during a tour of the explosion site. "These manhole covers were found completely blown off."
Attorney General Jesus Murillo Karam told Radio Formula Tuesday afternoon that investigators were still trying to determine if the gas came from an industrial leak or a naturally occurring buildup. Although most natural gas has an additive that makes it easy to smell, Murillo said that the methane from an industrial leak in the Pemex building would have been odorless, without explaining why.
Domestic and industrial natural gas is typically 90 percent methane, said Isaac Zlochower, a research chemist at the U.S. Centers for Disease Control and Prevention who has studied explosions of methane and other flammable gases but was not involved in the probe of the Pemex blast. If concentrations of methane in the air are 5-16 percent, an ignition source can cause an explosion.
"The issue is primarily to ensure that you don't have a flammable gas air mixture in a confined space," he said. "It's much harder to ensure that there isn't an ignition source."
Murillo said an independent contractor had told investigators that he was working with a crew of three men performing maintenance in the basement of building B2 on Thursday afternoon. 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 strong, sharp whistling through the corridor, coming from the area of the foundation pilings that were being worked on, and then right away he felt a strong explosion that threw him against the wall," Karam said.
The three men were found dead in the lower basement with burn marks, one with a fragment of cable stuck to his body.
Castillo said the maintenance supervisor reported that his crew had not been in the lower basement to inspect the foundations in seven or eight months. It was not immediately clear if Pemex, which is responsible for inspecting its own buildings, required more regular maintenance. A spokesman did not answer repeated calls Tuesday.
Murillo said investigators were still reviewing records of building inspections to determine why Pemex had not discovered the gas accumulation.
After days of speculation that the building had been bombed, Murillo said Mexican, Spanish, U.S. and British investigators looking into the blast found no evidence of explosives.
Murillo described a "diffuse" blast that 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.
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 had been set intentionally.
Murillo said there is not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.
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.
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