A blast that killed at least 25 people and injured about 100 at Pemex, Mexico's state oil company, ripped a building "from the inside out," a paramedic said.
Hours after the explosion Thursday at the office complex in Mexico City, rescuers searched the rubble for anyone trapped inside, CNN reported Friday.
One emergency worker said crews could smell "a strong odor of gas" when they entered the building.
At least 14 of the injured were hospitalized, two in serious condition.
Emergency responders said the explosion, which came from the basement, collapsed two floors of a building adjacent to the company's office tower.
The explosion tore through the building "from the inside out," paramedic Carlos Javier Rodriguez Jurado said.
Rescuers found many people trapped.
At least one person was rescued more than 5 hours after the blast, Mexican President Enrique Pena Nieto said on his Twitter page.
"People were screaming. ... You could see pieces of the wall falling to the ground," said Joaquin Borrell Valenzuela, an attorney with the Pemex comptroller's office, who was outside the building when the blast occurred.
The explosion was in an annex building just to the east of the tower, Foreign Minister Jose Antonio Meade said.
Officials evacuated the complex, which has been closed until further notice.
Thousands of people work at the Pemex headquarters, which includes a 54-story building that is one of Mexico City's tallest skyscrapers.
"We will get to the bottom of the causes in close coordination with the authorities," Pemex Director General Emilio Lozoya Austin said. "At this time, attending to the injured is the priority."
Mexico's attorney general's office will participate in the investigation of the explosion, officials said.
Pemex, short for Petroleos Mexicanos, was established after Mexico nationalized foreign oil operations in 1938.
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