The explosion that killed 37 people at the headquarters of Mexico's state-owned oil firm was caused by a gas build-up, officials said Monday, 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 skyscraper in the capital last 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, Attorney General Jesus Murillo Karam told a news conference.
The gas came into contact with a spark, causing a blast that brought down floors, killing 37 people, he said.
"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, which has suffered deadly industrial accidents as recently as last year.
The four-day delay in announcing the cause of the blast fueled conspiracy theories and criticism of the government's silence up to now.
"It is not possible that four days after this tragedy, accident and/or attack, the federal and city authorities have not been able to identify the cause," El Universal newspaper columnist Ricardo Aleman wrote on Monday.
He noted that the rumors ranged from negligence to an attack by either a drug cartel or a guerrilla group.
"From this wave of speculation, an extremely worrisome question comes up: What are they hiding?"
Mexico is in the throes of a drug war that has killed more than 70,000 people since 2006, with gangs fighting each other or authorities trying to catch them.
The cartels have been known to puncture Pemex pipelines to sell the fuel in the black market.
In December 2010, an oil pipeline exploded after it was tapped by thieves in the central town of San Martin Texmelucan, leaving 29 people dead and more than 50 injured.
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.
Earlier, in October 2007, 21 Pemex workers died during a gas leak on an oil platform in the Gulf of Mexico. Most drowned when they jumped into the sea in panic.