The cause of a deadly explosion at the headquarters of Mexico's state-owned oil firm remained shrouded in mystery on Friday, with authorities investigating if it was an accident or an attack 24 hours later.
As the toll jumped to 33 dead and 121 injured, government and Pemex company officials had yet to pinpoint what was behind the blast that ripped through the annex of the firm's Mexico City skyscraper on Thursday afternoon.
The blast erupted amid a debate over plans by President Enrique Pena Nieto to modernize Pemex and attract more outside investments to the old state monopoly, which has suffered deadly industrial accidents as recently as last year.
"The government is determined to find the truth in this incident, whatever it is; whether it was an accident, whether it was carelessness, whether it was an attack," attorney general Jesus Murillo Karam told a news conference.
"We don't want to leave anything to the imagination," he said as rescuers wound down the search, with rescuers focusing on two final locations. The last body was pulled out of the rubble at around noon.
Earlier, Pemex director general Emilio Lozoya Austin said the explosion appeared to have been an accident, though he insisted that all lines of investigation remained open.
"It appears that this is what one can observe as part of what experts refer to as an accident," he told the Televisa network.
A civil protection spokesman said on Thursday that witnesses had reported a gas build-up in an electricity supply room, but it was unclear whether it was the source of the disaster.
Pemex had indicated before the blast was confirmed that the building was evacuated due to an electrical failure.
Murillo Karam said there was no evidence of fire in the victims or debris. Investigators from the army, navy, federal police, prosecutor's office and two foreign firms were involved in the probe.
Hundreds of firefighters, police and soldiers aided by dogs dug through rubble for almost 24 hours straight, with the help of floodlights and cranes after the blast caused the mezzanine of the annex to collapse.
Thousands of people work in the Pemex complex, but officials said the area hit by the blast has four levels and housed 200 to 250 employees.
The government said two more locations were to be searched after they sifted through 39. Mexican Red Cross coordinator Isaac Oxenhaut had indicated earlier that the mission to rescue survivors or search for bodies was over.
"We did a sweep with other organizations, we brought dogs again," he told reporters. "We rule out there being any trapped victims."
Soldiers spent the day clearing mounds of debris from the area, which was strewn with piles of concrete, computers and office furniture.
The Mexican Congress held a minute of silence while Pemex said 52 people remained in hospitals.
Officials stressed that the blast will not interrupt production at the oil giant, the world's fourth-largest crude producer with an output of around 2.5 million barrels per day.
David Shields, a Pemex expert and author of the book "Pemex: The Oil Reform," said whatever the investigation determines will influence the debate over the company's future.
"If it was an attack, the repercussions will be on national security. If they determine that it was a maintenance problem, they will have to establish if it was the failure of the union or a contractor," Shields said.
Pena Nieto has not given details about his plans for Pemex, but he insists it will not be privatized.
The company has experienced deadly accidents at its oil and gas facilities in the past. Last year, a huge explosion killed 30 people at a gas plant near the northern city of Reynosa, close to the US border.
The previous worst incident took place in December 2010, when an oil pipeline exploded after it was punctured by thieves in the central town of San Martin Texmelucan, leaving 29 dead and injuring more than 50.
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.