An explosion caused the lower floors of a building to collapse at the headquarters of Petroleos Mexicanos (Pemex), the Mexican state-owned petroleum company, in Mexico City on Thursday. At least 33 were killed and 121 injured in the blast, which authorities believe may have been an accident.

According to reports, the explosion occurred in the basement of the B2 Building, a seven-story structure that served as an annex and was adjacent to Pemex’s executive tower, which is more than 50 stories tall. The blast destroyed most of the annex’s first and second floors but did not cause the building to collapse.

Pemex reportedly said in its Twitter feed that the explosion could have been caused by an electrical failure or a problem with a natural gas line. Its website identified 30 of the victims.

Hundreds of rescue workers were reportedly at the site on Friday, sifting through the rubble with dogs trying to find survivors.

“Petroleos Mexicanos is deeply shocked by the tragedy happened yesterday,” CEO Emilio Lozoya said in a translated statement. “Personally, I feel very shocked…I would like to express my heartfelt condolences for the dead and my solidarity with their families.”

According to Lozoya, of the 121 people who were injured in the blast, 52 remained hospitalized. He said between 200 and 250 people worked in the B2 Building.

Lozoya said officials with the Mexican federal government promised to use all resources available to determine the cause of the blast. “The investigation is continuing, and we believe we will come to a conclusion after the results come in.”

“Ladies and gentlemen, from this tragedy Pemex and its workers will come out stronger. Today Pemex workers are more united than ever. We honor the memory of our colleagues who died and offer our support and attention to the injured.”

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