(CBS)-- Rescuers searched for survivors Friday and authorities promised a thorough investigation after an office building blast killed 32 people and injured 121 at the headquarters of Mexico's state-owned oil company, Petroleos Mexicanos.
The cause of the basement explosion in an administrative building next to the iconic, 51-story Pemex tower in Mexico City remained a mystery, with President Enrique Pena Nieto urging people not to speculate. Theories ranged from an electrical fire to an air conditioning problem to a possible attack.
"We have no conclusive report on the reason," Pena Nieto told reporters. "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."
Pemex Director Emilio Lozoya said Friday that the death toll had risen to 32, with 121 people were treated for injuries after Thursday's blast. He said that rescue efforts continue and people still could be trapped.
More than 500 firefighters, soldiers and rescue workers dug through chunks of concrete with dogs, trucks and a Pemex crane.
Interior Minister Miguel Osorio Chong said it was uncertain if any of the roughly 10,000 people who work in the five-building headquarters were still trapped, but that the search would continue. The explosion occurred at about 3:45 p.m., just as the administrative shift was about to end. It hit the basement and first two floors, which rescuers said all collapsed onto each other.
"There is a lot of risk," rescuer German Vazquez Garcia said of working on the site.
Pemex first said it had evacuated the tower and 14-story administrative building because of a problem with the electrical system. The company later tweeted that the Attorney General's Office was investigating the explosion.
Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia Garcia, 36, who works in the building where the explosion occurred. She said she last talked to him a couple hours earlier.
"I called his phone many times, but a young man answered and told me he found the phone in the debris," Vargas 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."
Gabriela Espinoza, 50, a Pemex secretary for 29 years, was on the second floor of the tower when she said she heard two loud explosions and a third smaller one.
"There was a very loud roar. It was very ugly," she said.
Espinoza's co-worker, Tomas Rivera, 32, worked on the ground floor where the explosion occurred and said the force knocked him to the basement, fracturing his wrist and jaw. The injured were taken to two Pemex hospitals and other facilities, including the Red Cross hospital in the Polanco neighborhood near the oil company's office headquarters, where relatives huddled in the waiting room for news of their loved ones. Some walked out of meetings with the hospital social worker joyous, while others came out crying.
"We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows," said Maria Concepcion Andrade, 42, who lives on the same block as the Pemex building. "People started running from the building covered in dust. A lot of pieces were flying."
Streets surrounding the building were closed as evacuees wandered around, and rescue crews loaded the injured into ambulances.
Pemex, created as a state-owned company in 1938, has nearly 150,000 employees and in 2011 produced about 2.5 million barrels of crude oil a day, according to its website, with $111 billion in sales. Pena Nieto, who took office in December, has made Pemex reform the center of his platform, with a plan to pump new investment into a company whose profits feed much of Mexico's federal budget, but which has fallen behind other oil companies in production, technology and exploration.
Shortly before the explosion, Operations Director Carlos Murrieta reported via 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 center in northeast Mexico near the Texas border killed 30 workers in September, the largest-single toll in at least a decade for the company.