Rescuers have been working to find survivors trapped after a deadly blast at the Mexico City headquarters of the state oil company, Pemex.
Some 500 rescuers helped by dogs are searching the building for people believed missing after the explosion that killed 32 people and injured 100.
Relatives of employees have gathered in search of information - some trying to reach loved ones via mobile phone.
The cause of the blast is under investigation, Pemex says.
Pemex boss Emilio Loyoza said the investigation was "complicated", but would not rule out an attack.
He said 20 women and 12 men were killed in the blast, while 52 of 121 others who were treated for their injuries remained in hospital.
There were indications that some people remained under the rubble, but it was unclear how many, he added in a Friday news conference.
'Full weight of the law'
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.
One distraught relative, Ana Vargas Palacio, told Associated Press she had spoken to her husband, who works in the building, two hours before the blast.
"I called his phone many times, but a young man answered and told me he found the phone in the debris," Ms Vargas said.
Police cordoned off the streets around the building, which is located in a busy commercial area of Mexico City.
Rescuer German Vazquez Garcia told AP there was "a lot of risk" involved in the search.
President Enrique Pena Nieto and Mexico City Mayor Miguel Angel Mancera, who visited the scene of the blast, said Pemex rescue and security teams were working alongside city authorities to help the injured.
Mr Pena Nieto said: "We have no conclusive report on the reason [for the blast]. 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, that we apply the full weight of the law against them."
The BBC's Will Grant, at the scene, 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.