Rescuers are searching the rubble for survivors after an office building explosion killed at least 32 people and injured more than 100 at the headquarters of Mexico's state-owned oil company.
Emilio Lozoya, the head of oil giant Pemex, said on Friday that the death toll from the powerful explosion had risen to 32. Earlier in the day, the company had put the death toll at 25.
Lozoya said 121 people were treated for injuries in the blast as emergency workers continued to work on the site looking for more people trapped in the rubble.
Investigators also sifted through shattered glass and concrete at the bottom of the building in the early hours of Friday to try to find what caused the blast.
Authorities promised a thorough investigation of the basement explosion in an administrative building next to the iconic, 51-story Pemex tower in Mexico City.
President Enrique Pena Nieto urged people not to speculate. "We have no conclusive report on the reason," he told reporters after visiting the scene.
"We will work to get to the bottom of the investigation to find out, first, what happened, and if there are people responsible in this case, we will apply the full weight of the law against them.''
A spokesman for the civil protection agency said there was an apparent "accumulation of gas" in an electrical supply room.
It was not known how many people could be trapped inside the building.
Thursday's explosion sent shocked employees pouring out of the skyscraper beneath a pillar of black smoke, some carrying wounded people out on office chairs.
Survivors described an earthquake-like rumble that shook the floor.
"It was dramatic. The building was shaking and suddenly there was debris. We couldn't even see the people next to us," Pemex employee and union member Cristian Obele told reporters.
Pemex, the world's fourth-largest producer of crude with around 2.5 million barrels per day, announced earlier that it had evacuated the building due to a power failure.
The company has experienced deadly accidents at its oil and gas facilities in the past.
A September fire at a Pemex gas facility near the northern city of Reynosa which killed 30 people.
More than 300 were killed when a Pemex natural gas plant on the outskirts of Mexico City blew up in 1984.
Eight years later, about 200 people were killed and 1,500 injured after a series of underground gas explosions in Guadalajara, Mexico's second biggest city. An official investigation found Pemex was partly to blame.