MEXICO CITY — Mexican officials’ assertion that a buildup of natural gas – and not a bomb – caused last week’s devastating explosion at the Mexico City headquarters of the state oil company raised as many questions as answers on Tuesday, with at least one energy analyst voicing open skepticism of the explanation.

“I’ve never seen a building explode from this kind of accumulation of gas. Show me one of this magnitude,” said David Shields, an energy analyst based in Mexico City who edits a magazine called Energia a Debate (Debating Energy).

The massive blast Thursday destroyed four stories of a 13-story building, killed 37, injured 126, and intensified interest in a dismal safety record for Petroleos Mexicanos, widely known as Pemex.

In a Monday evening press conference, Attorney General Jesus Murillo Karam said methane leaked from a small outbuilding into the basement of an auxiliary office building next to a high-rise that is the center of the Pemex complex.

Murillo said investigators still don’t know how the leak occurred or why the methane ignited. But he ruled out any other cause for the blast.

The explosion did not char material like papers at the site, did not leave a crater, and did not rip apart bodies from the concussive blast, all typical of bombs, he said.

Showing floor plans, the attorney general said the blast originated in a basement level of building B-2, where a four-member team from a private firm was checking on underground pylons.

“The foreman stated that he was with these workers, but moments before the explosion he moved to another point,” Murillo said. There, the foreman heard “a loud and short whistle” coming from below “and immediately felt a strong blast that knocked him against the wall.”

Murillo ridiculed a report in La Jornada, a leftist newspaper, saying that relief workers found a suitcase near the site that contained undetonated explosives.

Authorities did find such a black roll-on suitcase, Murillo said, and they carefully transported it to a safe area in case of detonation.

“They opened it and found something that is very dangerous for a man: women’s cosmetics. And that was the most dangerous thing in the bag,” he said.

Shields questioned how workers without masks could be working in an unventilated basement filled with methane.

“The big question is about where the gas came from and how these guys survived in a gas-filled environment,” Shields said.

Some suspicion lingered after the government explanation. Alexia Barrios, a columnist for the digital SPD Noticias website, wrote Tuesday that the culprit of the blast could be called “the Lone GASsassin.”

Of the accumulated gas theory, she wrote, “no one believes this 100 percent.”

Pemex has a decades-long history of fatal accidents in which the oil giant never fully explains the causes. The most recent example is an explosion at a gas facility in September in Reynosa, along the border with Texas, that left 30 people dead.

The only other tragedy to hit the administrative headquarters in Mexico City directly was arson on Sept. 6, 1982, in the same B-2 building hit by last week’s blast. No one was hurt in the fire, but it consumed thousands of documents related to graft in the purchase of two ships. Two Pemex employees later fled to Chile in the wake of an investigation, which indicated that the fire was set to burn evidence.

President Enrique Pena Nieto on Tuesday hailed relief workers who attended victims of last week’s blast and pledged to deepen an investigation into its causes.

“I’ve reiterated to the attorney general to continue the inquiry until there is full certainty of what happened on Jan. 31,” Pena Nieto said at a ceremony in Queretaro, a manufacturing hub north of the capital.

In a conference call to energy industry executives and analysts, Pemex authorities said Tuesday that workers at the complex, who number as many as 10,000 on a normal workday, would return to their jobs Wednesday.

“We’re checking any potential accumulation of gases. We have none of that in other buildings. Our concern is to operate in a secure environment,” said Carlos Murrieta Cummings, director of operations for Pemex.

Chief financial officer Mario Beauregard Alvarez said the accident would not affect controversial plans to push through reforms later this year to open the energy sector to foreign investment. Mexico’s oil industry has been nationalized since 1938.