Last Updated 6:27 p.m. ET
MEXICO CITY 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.
"There is a lot of risk," rescuer German Vazquez Garcia said of working on the site.