The Mexican government said last Thursday’s explosion at the headquarters of state-owned petroleum company Petroleos Mexicanos (Pemex) in Mexico City was caused by a methane gas leak.

According to reports, at least 37 people have died from an explosion in the basement of the B2 Building, a seven-story structure that served as an annex and was adjacent to Pemex’s 52-story executive tower. The blast destroyed most of the annex’s first and second floors, but did not cause the building to collapse.

“We have been able to determine that the explosion was caused by an accumulation of gas in the basement of the [B2] building,” Mexico Attorney General Jesus Murillo Karam said, adding that the gas was believed to be methane. “This caused a defect in the structure of the floors, which first were pushed upward, and then fell, which was the main cause of death in the building.”

Murillo said investigators from Mexico, the United States, Britain and Spain had found no traces of explosives at the site, ruling out the possibility that the blast was an act of terrorism. But he said criminal charges could still be filed.

Authorities are reportedly trying to determine the source of the methane gas; several ducts, tunnels and a pipeline run underneath and adjacent to the B2 Building. A storage facility is also in the vicinity, and the entire city has been built atop a dry lake, which sometimes causes sewage problems.

Reuters reported that Murillo said a group of contractors were working on supports under the building at the time of the explosion, and needed electricity. They had reportedly used an extension cord, which could have caused a spark to ignite the methane gas.

Pemex CEO Emilio Lozoya said the company’s workers would return to work on Wednesday but the B1 and B2 buildings would remain closed.

The explosion is the deadliest disaster to hit Pemex since Sept. 18, 2012, when 30 workers at a natural gas compressor station and distribution center in Reynosa — in Tamaulipas state, just south of the U.S.-Mexico border at McAllen, TX — were killed in a fire (see Daily GPI, Sept. 21, 2012).

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