Twenty-five people are dead and at least 100 injured after an explosion at the headquarters of Mexican oil company Pemex.
Some 46 people remain in hospital following the blast at the state-owned firm which blew out windows and damaged three floors of the 52-storey skyscraper in Mexico City's commercial centre.
There are reports that as many as 30 people could be trapped in the debris from the explosion, which occurred in the basement of an administrative building next to the tower, where thousands of people work.
Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia Garcia, 36, who works in the building. She last heard from him at 1pm.
"I called his phone many times, but a young man answered and told me he found the phone in the debris," she said.
The two have an 11-year-old daughter. His mother, Gloria Garcia Castaneda, collapsed on a friend's arm, crying, "My son. My son."
Television images showed people being carried out of the building on office chairs. Most of them showed injuries likely to have been caused by falling debris.
Pemex - full name Petroleos Mexicanos - said in a tweet that several workers were injured in the blast but no one answered at its offices.
There was no immediate cause given for the blast, but in an earlier tweet, the company said it had evacuated the building because of problems with the electricity.
"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris," employee Cristian Obele told Milenio television, adding that he had been injured in the leg.
President Enrique Pena Nieto urged people not to speculate.
"We have no conclusive report on the reason," he said, pledging a probe "to get to the bottom" of the blast.
The main floor and the mezzanine of the auxiliary building, where the explosion occurred, were heavily damaged, along with windows as far as three floors up.
Interior ministry spokesman Eduardo Sanchez told Milenio: "Right now they're conducting a tour of the building and the area adjacent to the blast site to verify if there are any still trapped so they can be rescued immediately."
Maria Concepcion Andrade, 42, who lives on the block of Pemex building, said: "We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows.
"People started running from the building covered in dust. A lot of pieces were flying."
Police landed four rescue helicopters to remove the dead or injured. About a dozen tow trucks were furiously moving cars to make more landing room for the helicopters.
Streets surrounding the building were closed as evacuees wandered around.
Shortly before the explosion, Pemex operations director Carlos Murrieta said on Twitter that the company had reduced its accident rate in recent years. Most Pemex accidents have occurred at pipeline and refinery installations.
A fire at a pipeline metering centre in northeast Mexico near the Texas border killed 30 workers in September, the largest-single toll in at least a decade for the company.