Rescue work after a deadly explosion at the headquarters of the Mexican oil company Pemex entered its final and most dangerous phase, the attorney general has said.
Jesus Murillo Karam said there was risk of structure collapse in the last two of the 39 sectors of the affected area.
Authorities say no hypothesis for the blast that on Thursday killed 33 people and injured 121 has been ruled out.
Meanwhile, President Enrique Pena Nieto declared three days of mourning.
He offered his condolences to the families of the victims.
Hundreds of rescuers helped by dogs worked overnight and during most of the Friday searching the building in Mexico City for people believed missing after the explosion.
Speaking to reporters, Mr Murillo said that although the last person rescued from the debris had been found at noon on Friday (1800 GMT) work would continue until all the areas had been cleared.
"We will not stop. The teams are now in the most dangerous area," he told journalists.
He also said the investigations about the cause of the blast will be transparent.
The government is determined to find out the causes of the blast "whatever they are: accident, imprudence, attack, whatever," he said.
"We want to investigate all possible theses."
A number of expert teams from the government, military, universities and international institutions are reportedly working at the scene.
Earlier, the head of Pemex, Emilio Lozoya said the explosion was likely to have been an "accident", adding that no rule out any line of investigation had been dropped.
Thursday afternoon's explosion in the lower floors of a building adjoining the 54-storey Pemex skyscraper happened as shifts were changing, making the area particularly crowded.
Debris from the blast spread out on to the street in front of the building.
Red Cross ambulances and helicopters were shuttling the injured to hospital, while firefighters and soldiers dug through chunks of concrete with a crane to reach trapped survivors.
Police cordoned off the streets around the building, which is located in a busy commercial area of Mexico City.
The BBC's Will Grant says this is the biggest explosion to hit Mexico City for 30 years.
Pemex says its operations will continue to run normally - and commercial and financial obligations will continue to be met - despite the blast.
The company has experienced a number of fatal accidents in recent years.
Last September, 30 people died in an explosion at a Pemex gas plant in northern Mexico, thought to have been caused by a build-up of gas.