Mexico says Pemex blast caused by gas build-up, not explosives

MEXICO CITY, Feb 4 (Reuters) - A deadly blast that killed at least 37 people at Mexican state oil monopoly Pemex's headquarters in Mexico City was caused by a build-up of gas, the government said on Monday.

Attorney General Jesus Murillo said no trace of explosives was found at the site of the explosion, the latest in a string of safety lapses to hit the oil monopoly. New President Enrique Pena Nieto is seeking to push through a major overhaul of Pemex.

"We have been able to determine that the explosion was caused by an accumulation of gas in the basements of the building," Murillo told a news conference. He said 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," he added.

Thursday afternoon's blast at a building next at the Pemex complex in downtown Mexico City prompted speculation the incident could have been an act of sabotage. The explosion sparked renewed criticism of the oil giant's safety record.

For years a source of national pride, Pemex has proven stubbornly resistant to change. The company has become a touchstone for Mexico's capacity for economic reform since oil output began to lag the performance of other major producers.

A symbol of Mexican self-sufficiency since president Lazaro Cardenas expropriated U.S. and British oil companies in 1938 and nationalized the oil industry, Pemex has also become a byword for inefficiency and graft. (Reporting by Lorena Segura; Writing by Simon Gardner and Dave Graham; Editing by Eric Walsh)