MEXICO CITY —  A deadly explosion at the headquarters of Mexico’s national oil company was caused by a buildup of gas in the building’s basement.

Attorney-General Jesus Murillo Karam said an investigation found no evidence of explosives in the blast that killed 37 people and injured 120 others.

Investigators believe that a spark from an electrical fault detonated leaking methane gas.

The blast ripped through four floors of a headquarters administrative building and jolted the next-door 54-story Pemex Executive Tower, an emblematic feature of the Mexico City skyline, at almost exactly the hour workers were finishing a shift, a circumstance that undoubtedly increased the number of casualties.

The dead included a 9-year-old girl who had gone to work that day with her dad, and 20 women, possibly employees of the hard-hit human resources department.

The timing of the tragedy could hardly be worse for the powerful, unwieldy and struggling company, and especially for the pledged efforts of new Mexican President Enrique Peña Nieto to turn Pemex around and attract outside investment.

Pemex, though a 75-year-old symbol of Mexican nationalistic pride, is also widely seen as a hidebound institution with declining production that lags far behind the international industry standard.

“The whole thing is very damaging for Pemex’s image,” said David Shields, an independent energy analyst in Mexico City. “No matter what happened, it was inside the corporate headquarters. If it was a bomb, how did it get inside? If it was gas or a boiler explosion, how did that happen?”

Security is especially loose at the Pemex complex, just west of downtown Mexico City, Shields noted. People who walk in are screened, but cars driven into the parking bays are not checked. The blast is believed to have originated in a basement garage.

Pemex has chalked up a long string of nearly annual accidents, not always the firm’s fault. The deadliest include a 1984 explosion and fire at a Pemex gas plant on the outskirts of Mexico City, which killed an estimated 500 or more people (an exact toll was never disclosed); and a 1992 event in Guadalajara, in which about 200 were killed when an underground gasoline leak caused streets to literally blow up.

In September, 31 people were killed at a gas plant fire in the Tamaulipas city of Reynosa.

The cutting of pumping lines and siphoning of gas or oil, sometimes by drug traffickers, are common occurrences throughout the Pemex system, and the acts sometimes ignite fires or create leaks that force massive evacuations of area residents.

In 2007, leftist guerrillas bombed numerous pipelines, and other radical groups employing similar tactics have targeted Pemex over the years, though nearly always in the remote areas of the company’s production.