The death toll in a mystery explosion at the headquarters of Mexico's state-owned oil giant Pemex rose to 32 on Friday as rescuers dug through mounds of rubble for survivors.
Hundreds of firefighters, police and soldiers toiled through the night after the blast ripped through an annex of the 54-floor tower, injuring 121 people and leaving concrete, computers and office furniture strewn on the ground.
Pemex director general Emilio Lozoya Austin said 20 women and 12 men died in the incident, while 52 more people remain hospitalized. He said the search for survivors would continue.
Lozoya Austin added Mexican and foreign experts were investigating the cause of the tragedy and that "we won't speculate, we won't get ahead of ourselves."
The blast will not interrupt production at Pemex, the world's fourth-largest crude producer with an output around 2.5 million barrels per day, he said.
Survivors described an earthquake-like rumble that shook the floor and shattered windows.
The blast heavily damaged the ground floor and mezzanine of the annex, and witnesses said a roof connecting the annex to the tower collapsed.
The area hit by the blast has four floors and houses 200 to 250 employees, Lozoya Austin said.
Mexican Red Cross national coordinator Isaac Oxenhaut said rescuers will scour the site "centimeter by centimeter until we are absolutely sure that no one is in there."
The area is "dangerous to work in," he said, adding that the search could be completed by the end of the day.
At least six ambulances were at the scene in case any people were found, while police partially reopened traffic on the heavily-traveled avenue in front of the complex.
"We were waiting all night to assist in a major emergency that did not materialize because, fortunately, it appears that almost everybody was taken out," a military nurse who refused to give her name told AFP.
Floodlights shined on the rubble and two cranes were brought to help rescuers in hard hats and surgical masks look for survivors.
One was found almost six hours after the blast, which took place around 3:40 p.m. (2140 GMT).
A spokesman for the civil protection agency said Thursday that there was an apparent "accumulation of gas" in an electrical supply room, but the exact cause of the blast has yet to be confirmed.
President Enrique Pena Nieto visited the site and survivors at the hospital late Thursday.
Interior Minister Miguel Angel Osorio Chong warned against speculation, saying Thursday that the goal of the probe was to "produce precise, trustworthy and convincing data to find out the origin and cause of the disaster."
The explosion sent shocked employees pouring out of the complex beneath a pillar of black smoke, some carrying wounded people out on office chairs in a city accustomed and equipped to handle earthquakes.
"We had two minutes to leave the building. I was headed to the pharmacy when the windows broke. It was a deafening noise," Astrid Garcia Trevino, who worked in the annex, told AFP. "The floor shook as if it was an earthquake."
Gloria Garcia said her brother Daniel, 35, had called from the building and said he was trying to get out. She hasn't heard from him since.
"We're afraid he might still be in there," she said.
Pemex had indicated before the blast was confirmed that the building was evacuated due to an electrical failure.
Pena Nieto took office in December promising to modernize Pemex in order to attract more private investment, but he insists that the company will never 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.