A gas build-up caused the explosion that rocked the headquarters of Mexico's state-owned oil firm last week, killing 37 people, officials said Monday, ruling out a bomb attack.
The announcement came four days after the tragedy at Pemex, a delay that triggered much speculation about the cause of the blast at the firm's offices in the heart of Mexico City.
The explosion tore through a human resources building next to the company's skyscraper last Thursday afternoon, destroying lower floors, causing thousands of workers to flee into the street and 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, the main cause of death, he said. The type of gas has yet to be determined but it appears to have been methane, he added.
"We confirmed that there are no traces of explosives," he said, adding that the blast did not leave a crater and that the victims were not dismembered like in bombings.
Asked whether the gas leak could have been an attack or negligence, Murillos Karam said the investigation had yet to determine if anybody was at fault.
"We haven't finished the investigation. The part that is missing is finding who is responsible, if such is the case," he said.
The attorney general said the team of investigators included officials from the army, marines and police as well as US, British and Spanish experts.
The 37th victim died at a hospital on Monday. Rescuers had worked through the weekend to look for more victims in the rubble.
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 in the recent past.
The four-day delay in announcing the cause of the blast fueled conspiracy theories and criticism of the government's failure to come up with an explanation 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 on 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.
Pemex director general Emilio Lozoya Austin said the headquarters would reopen for work on Wednesday, except for the building that was hit by the explosion.