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.