Emilio Lozoya said, "It appears from what one can observe that this is part of what the experts view as an accident," the Los Angeles Times reported Friday.
He said no line of investigation had been ruled out.
At least 33 people have been reported killed and more than 120 injured by a blast that ripped through four floors of the company's building Thursday. An undetermined number of people were missing.
There was no fire after the blast, which occurred in an underground parking garage, senior officials said.
Mexican Attorney General Jesus Murillo Karam warned that emergency workers had entered the most dangerous phase of the work because of the risk of structural collapse in the last two of the 39 areas damaged by the blast, CNN reported.