An explosion at the headquarters for Petroleos Mexicanos last week left 37 people dead.
Mexican Attorney General Jesus Murillo said preliminary results of an investigation suggested a methane gas leak led to the explosion.
"An accumulation of gas that was combined with a spark from an electrical system in the building provoked this tragedy," he was quoted by Bloomberg News as saying.
Murillo said it was unclear whether the explosion was accidental or intentional.
"We still need to go through a lot of steps in order to determine the precise gas that generated the explosion, its exact point of origin and the analysis of whether there are people responsible for this," he said.
Analysts said the explosion wasn't likely to cause a major effect on Mexico's oil sector, one of the most productive in the world. Employees are expected to return to the headquarters Wednesday.
Mexican President Enrique Pena Nieto, in office since December, said he aims to lower taxes on the company to court private investments to an ailing energy sector.