A rescue member takes a break at the headquarters of Mexican oil giant Pemex in Mexico City on February 1, 2013
A blast that killed 33 people at the headquarters of Mexico's state-owned oil firm Pemex appears to have been an accident, the firm's chief said Friday as rescuers wrapped up a grueling search for survivors.
Hundreds of firefighters, police and soldiers aided by dogs dug through rubble for almost 24 hours straight after the explosion ripped through an annex of the 54-floor tower in the heart of Mexico City, injuring 121 people.
"It appears that this is what one can observe as part of what experts refer to as an accident. However, no line of investigation will be ruled out," Pemex director general Emilio Lozoya Austin told the Televisa network.
Earlier, Lozoya had refused to speculate on the cause of the blast when asked during a news conference whether the explosion was a deliberate attack. "We won't get ahead of ourselves," he said, adding that experts were on the case.
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.
Mexican Red Cross coordinator Isaac Oxenhaut said the search for survivors or 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 continued to clear mounds of debris from the area, which was strewn with piles of concrete, computers and office furniture.
Interior Minister Miguel Angel Osorio Chong said the death toll had reached 33, from 25 overnight. The Mexican Congress held a minute of silence. Pemex said 52 people remained in hospitals.
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 stressed that the blast will not interrupt production at oil giant, 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.
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 search for survivors went on all night, with floodlights and two cranes assisting rescuers in hard hats and surgical masks. One was found almost six hours after the blast, which took place around 3:40 p.m. (2140 GMT).
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.
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.
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.