MEXICO CITY (AP) -- Gas could have built up for decades before an explosion caused three floors of the headquarters of the national oil company to collapse, killing 37 people, Mexican federal prosecutors said Friday.
Methane gas combined with traces of hydrocarbons beneath the building triggered the explosion, the Attorney General's Office said in a statement.
Vapors from solvents used for cleaning also contributed to the blast, it said.
The office said that methane gas accumulated from fuel kept in the building by a gas company that in the 1930s used the same lot where the Petroleos Mexicanos, or Pemex, headquarters are now located. It said the gas beneath the surface could also have come from a fuel warehouse that was once there.
Methane is the main component of most natural gas used for cooking and heating.
The statement said the Jan. 31 explosion was ignited by either a mechanical or electrical spark. Investigators found an extension cord, a lamp and a plug that could have ignited the blast, it said.
The explosion earlier this year was the petroleum giant's worst disaster in a decade.
Mexico's government relies on oil revenues for about a third of its budget.
President Enrique Pena Nieto has said he will present a bill to Congress next week to allow for more private investment for the company to be more competitive.