Last week's deadly explosion at the headquarters of Mexico's state-owned oil company was caused by a gas buildup.

Attorney General Jesus Murillo Karam told reporters Monday that an investigation into the disaster found that electrical spark had detonated an accumulation of gas in the basement of the Pemex building. Murillo said the gas appeared to be methane.

At least 37 people were killed in last Friday's explosion, which caused the building to collapse.

Murillo said there were no traces of explosives at the blast site, such as a large crater typically caused by a bomb.

The investigation was carried out by an international team of specialists from Mexico, Spain, Britain and the United States.

Once a symbol of national pride, Pemex has been plagued by a string of accidents in recent years, as its facilities have gone neglected. Mexico's new president, Enrique Pena Nieto, has proposed a major overhaul of the oil monopoly and is seeking to attract private investment to upgrade its infrastructure.