San Juan, Puerto Rico (CNN) -- A fire at a fuel storage facility that burned for three days and forced hundreds of Puerto Rico residents from their homes has been extinguished, fire officials said Sunday evening.

But some of the tanks continued to smolder and crews stood ready to fight another outbreak.

Hundreds of firefighters battled the blaze, which began with an explosion at the Caribbean Petroleum Corp. facility in Puerto Rico's Bayamon municipality, near San Juan, early Friday. The initial explosion shook the ground with the force of a 2.8-magnitude earthquake, authorities said, and flames shot into the air while plumes of thick, black smoke hovered over the region.

As the fire died, reporters and officials were able to witness firsthand the extent of the damage. Seventeen tanks were destroyed by flames and the initial explosion.

Agents from the FBI and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were investigating to determine whether the explosion was an act of sabotage.

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." Caribbean Petroleum owns the Gulf Oil brand, but Rodriguez said he did not know what ACNF referred to.

Agents were at the storage facility and treating it as a crime scene, which is a routine designation.

Puerto Rico's governor, Luis Fortuno, said the main priority of his government is to counter any long-term effects of air and water pollution caused by the fire. Representatives from the Environmental Protection Agency had been checking air quality near the fuel depot.

"We're not finding levels that would be of concern," EPA spokeswoman Bonnie Bellow said.

She said the fire had been so intense that it was "burning off chemicals that are part of the fuel."

About 600 displaced people were in shelters on Sunday night, but many were being sent home. Dr. Lorenzo Gonzalez, Puerto Rico's health secretary, said everyone at shelters and all the emergency personnel were vaccinated against the H1N1 flu virus as a precaution, given their close proximity to each other.

He said tests have been conducted on the air and water, and the results showed no reason for concern.

President Obama declared an emergency in Puerto Rico, which frees up federal aid. Puerto Rico is a territory of the United States.

Fortuno said the blaze has cost the island at least $6.4 million.

The governor sought to allay fears over gasoline supplies. Caribbean Petroleum owns 200 gas stations on the island and several inland distribution facilities, and supplies much of the island's fuel.

Puerto Rico will receive 3.6 million gallons of regular gasoline, more than 1 million gallons of premium gasoline and more than 1 million gallons of diesel fuel to help make up for what may have been lost, Fortuno said.

The company has had violations of the federal Resource Conservation and Recovery Act, the main law in the United States that deals with the disposal of solid and hazardous wastes, according to EPA spokeswoman Bellow. Caribbean Petroleum is under a corrective-action plan, she said.



CNN's Rafael Romo in San Juan and Arthur Brice in Atlanta, Georgia, contributed to this report.