MEXICO CITY (AP) - A blast that collapsed the lower floors of a building in the headquarters of Mexico's state-owned oil company, crushing at least 33 people beneath tons of rubble and injuring 121, is being looked at as an accident although all lines of investigation remain open, the head of Petroleos Mexicanos said Friday. As hundreds of emergency workers dug through the rubble, the company's worst disaster in a decade was fueling debate about the state of Pemex, a vital source of government revenue that is suffering from decades of underinvestment and has been hit by a recent series of accidents that have tarnished its otherwise improving safety record. Until now, virtually all had hit its petroleum infrastructure, not office buildings. "It seems like, from what experts can observe, is that it was an accident," Pemex Director-General Emilio Lozoya told the Televisa network. However no line of investigation will be discounted. Mexican President Enrique Pena Nieto has pledged to open the oil behemoth to more private and foreign investment, setting off warnings among leftists about the privatization of an enterprise seen as one of the pillars of the Mexican state. Pena Nieto has provided few details of the reform he will propose but denies any plan to privatize Pemex. In a debate on MVS Radio about Pemex Friday morning, Juan Bueno Torio, a congressman from the conservative National Action Party, said Pemex should be granted more budgetary independence as part of the reform, allowing it to better address infrastructure problems that he said have been neglected under government control. "There are always maintenance problems," he said. These are definitely issues that we'll take up again when the new energy reform comes up. Manuel Bartlett, a senator from the leftist Workers' Party, shot back that Pena Nieto "has been touring the world inviting investors and foreigners to invest in Pemex." "Privatizing Pemex is taking away the control of the Mexican state and transferring it to private hands," he said. Less than 24 hours after the accident at Pemex headquarters, early signs pointed to a problem in an area with electrical and air-conditioning equipment, according to a government official who was not authorized to speak by name. Pemex said in a tweet in the first minutes after the accident it had evacuated the building because of a problem with the electrical system. A Pemex spokesman said the floors hit by the explosion housed administrative offices. Pemex and Interior Secretary Miguel Angel Osorio Chong said on their Twitter accounts Friday afternoon that the death toll had risen to 33. Mexico City Mayor Miguel Angel Mancera declared three days of mourning, with flags at half-staff and black bunting hung on public buildings. Lozoya said the priority remains rescue and recovery, plus attending to the injured and families of those who died. 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. Marco Franco Hernandez, a national rescue coordinator for the Mexican Red Cross, told The Associated Press that there was still a possibility of pulling a small number of victims out of the rubble, and rescuers were trying to maintain optimism that someone could be pulled out alive. The explosion was the worst in more than a decade for Pemex. Last September, an enormous blast killed 30 workers at a pipeline facility in northern Mexico. That disaster was a major setback to a safety record that had been improving following a series of incidents in the 1980s and 1990s, according to company figures. The number of accidents per million hours worked dropped by more than half, from 1.06 in 2005 to 0.42 in 2010. That is in line with the international average of about 0.43 per million, according to the U.K.-based International Association of Oil and Gas Producers, which does not independently verify company numbers. But Pemex acknowledged in a report that starting in late 2011, a series of smaller blasts and fires, mainly at refineries and petrochemical plants, had "seriously impacted" its safety rate. It said the rate of injuries per million hours had risen to 0.54. Mexico City-based oil analyst David Shields said he was pessimistic that any one particular accident would have a real impact on the broader push for reform. "What they really do is spark a certain amount of indignation, most of the time they don't result in changes," he said. At the end of the day, things don't seem to work out. Before the pipeline blast in Reynosa, Pemex's last big accident was in 2007, when a sudden storm hit an offshore oil rig, killing 22 workers. Thursday's 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-story administrative building at the headquarters office complex, which was built starting in the late 1970s. Some 10,000 people work there daily. Lozoya said about 1,700 work in the building affected. To the untrained observer, the offices appeared to be showing signs of age surprising for the headquarters of a major oil company. The elevators were often out of service or crept slowly between floors. Its bathrooms were dirty and the floors were worn. 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 neighborhood near the oil company's headquarters. Relatives huddled in the waiting room for news of their loved ones. 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. AP writers Katherine Corcoran, Olga R. Rodriguez and Adriana Gomez contributed to this report.