San Juan, Puerto Rico -- Hundreds of firefighters battled a blaze at a Puerto Rico oil storage facility Sunday that forced 1,000 residents from their homes.
Jose Figueroa Sancha, chief of police in Puerto Rico, told CNN that authorities expect to have the fire, now in its third day, contained by 6 p.m. In all, 350 firefighters were working to quash the flames, with about 1,000 police officers helping with security and traffic flow around the area, Sancha said.
One oil tank remained on fire Sunday morning.
Five tanks were burning Saturday night. An additional 12 tanks were smoking, but posed no danger. Four had burned themselves out and collapsed.
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.
However, it takes about two weeks after a vaccination for the antibodies providing protection to build up in the body, according to the Centers for Disease Control and Prevention (CDC).
Representatives from the Environmental Protection Agency were checking for contaminants in the air near the fire.
"We're not finding levels that would be of concern," said Bonnie Bellow, EPA spokeswoman. "The fire is so intense that it is burning off chemicals that are part of the fuel."
Gonzalez said tests have been conducted on the air and water, and the results showed no reason for concern.
The blaze at the Caribbean Petroleum Corp. facility in Bayamon municipality, near San Juan, started shortly after midnight Friday. The initial explosion shook the ground with the force of a 2.8-magnitude earthquake.
Sancha said it is too early to determine the cause, and the investigation won't begin until the fire is completely out.
Agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the storage facility and treating it as a crime scene, which is a routine designation.
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 or who ACNF is.
Investigators are not discounting any possibilities, said Orlando Felix, an ATF agent in Puerto Rico.
President Obama declared an emergency in Puerto Rico, which frees up federal aid. Puerto Rico is a territory of the United States.
Gov. Luis 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 in 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. "There's no reason to worry in the least."
The company has been charged with 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.