Mexico City, Mexico - The massive explosion that killed nearly 40 people in late January at state oil company Pemex's Mexico City headquarters was caused by a gas buildup, Mexico's attorney general's office said last Friday, ruling out foul play.

A six-month investigation confirmed a preliminary conclusion that the January 31, 2013 explosion that collapsed several floors of the headquarters' B2 building, which contained archived documents, was caused by faulty building design that provided poor ventilation in the basement area.

Methane from the soil and other gases collecting in the basement of the building for years with no ventilation, in addition to fumes from industrial solvents stored there, were accidentally sparked by workers attempting repairs, the investigation found.

The conclusion of the investigation comes as Mexico's Congress is about to launch a debate over energy reform legislation that aims to overhaul Pemex and the country's oil, gas, and electricity sectors.

President Enrique Pena Nieto's energy reform bill is expected to be presented to lawmakers this week.