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.