Investigators don't know if the massive fire at a fuel storage facility near San Juan was deliberately started or was an accident, the agent in charge of the FBI's San Juan office said Monday.

The blaze, which started with an explosion at the Caribbean Petroleum Corp. facility in Bayamon municipality early Friday, was extinguished Sunday, firefighters said. Some of the tanks continued to smolder and crews stood ready to fight any flare-up.

The FBI is investigating graffiti found on two San Juan tunnels that referred to a fire, Special Agent Harry Rodriguez said Friday. A spray-painted message on the tunnels, less than three miles apart, said: "Boom, fire, RIP, Gulf, Soul, ACNF."