Mexico's President Enrique Pena Nieto waves to the crowd while arriving at the headquarters of state oil giant Pemex in Mexico City, after a powerful explosion at the building killed at least 32 people. (Tomas Bravo/Reuters)

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-storey Pemex tower in Mexico City remained a mystery, with President Enrique Pena Nieto urging people not to speculate.

But early signs pointed to an accident in an area that housed electrical and air-conditioning equipment, according to a government official who was not authorized to speak by name.

Pemex director Emilio Lozoya said the same in an interview with Televisa, but added that no line of investigation is being discarded. Questions continued to abound about a possible attack on Mexico's most important installation just as the government is talking about a controversial energy reform and opening up Pemex to outside investors.

"We have no conclusive report on the reason," Pena Nieto told reporters late Thursday. "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."

Lozoya said the priority remains rescue and recovery, plus attending to the injured and families of those who died as the death toll had risen to 32, with 121 people treated for injuries after Thursday's blast. He said 52 remained hospitalized and survivors and bodies still may be found in the rubble.

More than 500 firefighters, soldiers and rescue workers dug through chunks of concrete, aided by dogs, trucks and a Pemex crane.

The explosion occurred at about 3:45 p.m., just as the administrative shift was about to end. It hit the basement and three floors, where as many as 250 people work, Lozoya said. The floors collapsed in the 14-storey administrative building at the headquarters office complex, where some 10,000 people work daily. Lozoya said about 1,700 work in the building affected.

"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 administrative building because of a problem with the electrical system. The company later tweeted that the Attorney General's Office was investigating the explosion.

'A very loud roar'

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 in the building 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 neighbourhood near the oil company's headquarters.

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.

Lozoya said Pemex operations continue uninterrupted despite the blast and that the company is producing 2.57 million barrels of oil per day.

Pemex, created as a state-owned company in 1938, has nearly 150,000 employees, according to its website, with $111 billion in sales. Pena Nieto, who took office in December, has made Pemex reform the centre 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 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

With files from CBC News