Rescue workers search for victims at the Pemex headquarters in Mexico City on Thursday.

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NEW: The exact source of the gas remains unclear

No traces of explosives were found, says an official

At least 37 people were killed in the blast

A deadly blast that killed at least 37 people at the headquarters of Mexico’s state-run oil giant was caused by a buildup of gas in the basement, an official said Monday.

Mexican Attorney General Jesus Murillo Karam told reporters that no traces of explosives were found at the scene.

“The cause is clear; it’s a gas explosion,” he said, adding that the gas in question is believed to be methane.

The exact source of the gas remains unclear. Investigations are ongoing, Murillo said.

At least 37 people were killed and more than 100 injured in the blast, which took place Thursday at an administrative building in the Pemex office complex in Mexico City.

Pemex is Mexico’s powerful and much-criticized state oil monopoly.

Of those injured, at least 28 remained hospitalized Monday night, according to the health ministry.

Thousands of people work at the company’s headquarters, which includes a 54-story building that is nearly 700 feet tall. The annex where the explosion happened is adjacent to the tower.

Employees at the annex said that the basement of the building, where the explosion originated, housed a water treatment facility, storage areas and archives. The first floor, which also was heavily damaged, is where the company’s human resources team worked.

A makeshift memorial for the victims has sprung up outside the complex, where handwritten notes, flowers and candles line a fence. contributed to this report.