MEXICO CITY — An explosion that collapsed the lower floors of a building in the headquarters of Mexico’s state-owned oil company, crushing at least 33 people to death 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 thathas tarnished its otherwise improving safety record.