Mexico's Attorney General's Office said the deadly January explosion at state oil company Petroleos Mexicanos' headquarters in this capital was caused by a mix of methane gas of biological origin and vapors from solvents.

Among the factors triggering the blast in Building B2's basement that killed more than three-dozen people, the AG's office pointed to the accumulation of methane over time and vapors from solvents used for maintenance.

The investigation found "electrical elements for igniting the material: a light cable extension, a lamp and a plug," it added.

Pemex has since installed ventilation systems in that building, which is adjacent to the main tower, with the aim of preventing the accumulation of methane gas and a repeat of the Jan. 31 tragedy, in which 37 people were killed and more than 100 were injured.

The geological analysis revealed that several layers of sediment were soaked with hydrocarbons, which may have come from crude spills by an oil company that occupied the site decades ago.

Micro-organisms that produce methane thrived due to the presence of the hydrocarbons and improper ventilation.

The methane gas filtered up from the soil and combined with the vapors from the solvents to produce an "explosive mix," the AG's office said. EFE