The Mexican government has vowed to find out if the explosion at the Mexico City headquarters of the state-run Pemex oil company that killed at least 33 people was an attack or an accident. More than 120 people were injured.
Attorney General Jesus Murillo said late Friday "the government is determined to find out the truth,...whether it was an accident, negligence or an attack."
Earlier Friday, the chief executive of the company said the blast appeared to be an accident but that the investigation remains open.
The blast late Thursday shattered the lower floors of the company's more than 50-story tower in the capital - one of the tallest buildings in Mexico City. Helicopters, firefighters and emergency technicians were quickly on the scene searching for survivors.
Mexican President Enrique Pena Nieto visited the site of the blast late Thursday. He offered his condolences to the families of the victims, and said he has ordered an investigation.
This is not the first deadly accident at a Pemex facility.
In September, a huge explosion at a Pemex plant near the city of Reynosa killed 30 people.
In 2010, a Pemex oil pipeline exploded in the central town of San Martin Texmelucan, killing 29.