MEXICO CITY -- The mystery intensified Friday over a huge explosion a day earlier at the headquarters of Mexico’s state-owned oil giant, a company vital to the nation’s economic health and at the heart of a fierce debate over energy resources.

The toll from the blast at mid-afternoon on Thursday climbed to 33 dead and 121 people injured, according to government officials.

Emilio Lozoya Austin, director general of Petroleos Mexicanos, or Pemex, as the company is commonly known, said in a brief interview with McClatchy that authorities had not discarded any possible avenue of investigation into the blast, but put off questions about whether it could have been sabotage.

“Speculating whether it is an accident or not is not my priority right now,” said Lozoya, who rushed back from a trip to Singapore after the explosion occurred. “What we need now is the solidarity of the community.”

The blast took place shortly before 4 p.m. local time in a landmark complex that comprises Pemex’s administrative headquarters. A high-rise tower of more than 50 stories is at the center of the complex, and three 13-story buildings and several others are adjacent. Some 10,000 people labor in the complex each workday.

It was in one of the 13-story buildings, known as B-2, where the blast shattered the afternoon calm as hundreds of employees returned from lunch or prepared to leave for the day. The explosion heavily damaged four floors of the B-2 building, where 200 to 250 people work, and “collapsed some of these floors, and therefore the loss of human life has been very important and tragic,” Lozoya said.

Lozoya said most Pemex workers would return to their jobs at the complex after a national holiday Monday.

“I want to be very clear,” he said. “The Pemex tower, the big building, is not affected at all. Pemex is operating, is producing 2.55 million barrels per day, more than 6 million cubic feet of gas. Our operations are up and running.”

Speculation on what triggered the blast ranged from a natural gas buildup or an electrical malfunction, to sabotage. Many Mexicans seemed doubtful a satisfactory explanation would ever be offered.

Mexico’s Chamber of Deputies held a minute of silence for the victims on Friday and issued a call for “the rapid explanation of the facts.”

In a sign of the jittery nerves following the lethal explosion, security experts swept the legislature, known as San Lazaro, for possible threats prior to the session.

Pemex, created in 1938 with the nationalization of foreign oil companies, is the second largest company in Latin America, after Brazil’s Petrobras.

It is a ubiquitous presence in Mexico due to its monopoly on retail sales of gasoline through its 46,000 or so stations. It is also a symbol of Mexican pride in state ownership of energy resources, until recently a third rail of politics.

But frustration is rising at inefficiency, poor maintenance and falling production at Pemex, which finances government spending through its earnings.

“Pemex can’t continue operating in the same way it’s been operating in past years,” said Alberto Islas, a security analyst with the crisis management firm Risk Evaluation, referring to lethal industrial accidents.

An explosion at a Pemex gas facility near the city of Reynosa on the Texas border killed 30 people in September, and no full report was ever released.