Rescuers dug through mounds of rubble for survivors on Friday after 32 people were killed in a mysterious explosion at the headquarters of Mexico's state-owned oil giant Pemex.
Hundreds of firefighters, police and soldiers aided by dogs have toiled since the blast ripped through an annex of the 54-floor tower on Thursday, 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. The Mexican Congress held a minute of silence.
As experts investigated the tragedy, Lozoya Austin refused to answer questions about whether an attack was to blame, saying "we won't speculate, we won't get ahead of ourselves."
A civil protection spokesman told AFP 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.
The blast erupted amid a debate over plans by President Enrique Pena Nieto to modernize Pemex and attract more outside investments to the state monopoly, which has suffered deadly industrial accidents as recently as last year.
Officials said 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.
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.
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.
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.
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.
A marine said Friday morning that rescue dogs had not detected any people in the rubble.
"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. The search focused on the basement.
One was found almost six hours after the blast, which took place around 3:40 p.m. (2140 GMT).
Interior Minister Miguel Angel Osorio Chong said Thursday 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.
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.