Last week's deadly explosion at the headquarters of Mexico's national oil company that killed 37 people was caused by a gas buildup in the building's basement, Mexico's attorney general said Sunday.

Attorney-General Jesus Murillo Karam said an investigation launched by Mexican, Spanish, American and British experts found no explosives in the blast that knocked out several floors of the Petroleos Mexicanos administrative building, according to the Associated Press.

The experts concluded an electrical fault caused a spark that detonated the leaking gas, causing a "diffuse" explosion that released a cloud of gas, rather than the an explosion usually seen from a compact source like a bomb.

From the Associated Press:

The announcement late Monday ended days of silence about the potential cause of the company's worst disaster in a decade. The blast fueled debate about the state of Pemex, a vital source of government revenue that is suffering from decades of underinvestment and has been hit by a recent series of accidents that have tarnished its otherwise improving safety record.

The gas explosion caused the structures of the floors to rise and then immediately fall, killing over 35 of the company's workers.

Workers described the blast as an "amazing explosion" that threw them from their desks. Many thought it was an earthquake and bolted down 10 flights of stairs after ceiling panels started falling out and glass started raining down form surrounding windows.

NPR reported that rescue crews pulled out three more bodies from the PMEX headquarters over the weekend.

Read the whole story here.

Watch a report from KTLA5 on the gas leak.

Read more of Neon Tommy's Pemex Explosion coverage here

Reach Editor At Large Brianna Sacks here