The explosion that killed 37 people at the headquarters of Mexico's state-owned oil firm last week was caused by a gas build-up, officials said, ruling out a bomb attack.
The announcement came four days after the blast, a delay that triggered much speculation about the cause of the blast at the complex that houses the offices of Pemex in the heart of Mexico City.
The explosion tore through an annex of the company's landmark skyscraper in the capital on Thursday, injuring more than 120 people.
"We were able to determine that the explosion was caused by an accumulation of gas in the basement" of the annex, said Attorney General Jesús Murillo Karam. The gas came into contact with a spark, causing a blast that brought down floors, killing 37 people.
"We confirmed that there are no traces of explosives," he said, adding that the blast did not leave a crater and that the victims did not have the type of dismemberment caused by bombs.
The attorney general said the team of investigators included officials from the army, marines and police as well as US and Spanish agents.
The blast erupted amid a debate over plans by President Enrique Pena Nieto to reform Pemex and attract more outside investments to the state monopoly.
The tragedy at the Pemex headquarters put a harsh spotlight on the company's history of deadly accidents.
In September, an huge explosion killed 30 people at a gas facility near the US border.
In October 2007, 21 Pemex workers died in a gas leak on an oil platform in the Gulf of Mexico.