More bodies were found in the debris at the headquarters of the state-run oil monopoly in Mexico City on Friday after an explosion that killed at least 33 people.
The blast occurred at a Pemex building next to the company's 50-story skyscraper on Thursday afternoon.
Officials are unable to say how many people may still be trapped in the wreckage. The death toll is expected to rise.
Pemex chief executive Emilio Lozoya said it was not clear what caused the explosion, which has been the subject of speculation ranging from a bomb, to a gas leak, to a boiler blowing up.
"A fatal incident like yesterday's cannot be explained in two hours, we are working with the best teams in Mexico and from overseas, we will not speculate," he said.
The US Bureau of Alcohol, Tobacco, Firearms and Explosives said a team was on its way to Mexico City at the request of the Mexican government. The team includes explosive specialists and fire experts.
Mr Lozoya said the four floors worst affected by the explosion normally had about 200 - 250 people working on them. That compared with about 10,000 people in the entire complex.
A BBC correspondent said the explosion was the biggest in Mexico City for 30 years.
In September, a fire at a Pemex gas facility near the northern city of Reynosa 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 1500 injured after a series of underground gas explosions in Guadalajara.
Big earner
Pemex was created in 1938 when the oil industry was nationalised. The BBC reports it employs 150,000 people and accounts for around 37% of government revenue.
President Enrique Pena Nieto took office in December saying that overhauling the company was a top priority.
