By MICHAEL WEISSENSTEINMEXICO CITY (AP) â¿¿ 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.