(CNN) -- A wind shift Saturday has put more residents at risk from smoke inhalation near a raging fire at a fuel storage complex in Puerto Rico, officials said.
Authorities also are concerned that expected precipitation Saturday mixed with the smoke could lead to acid rain. Residents were urged not to venture outside and to bring in their animals during the rain.
Firefighters, meanwhile, continued to battle the blaze 36 hours after it started with a shattering explosion that shook the ground with the force of a 2.8 magnitude earthquake. The blast was heard -- and felt -- miles away.
At least 15 of the 40 tanks at the Caribbean Petroleum Corp. facility in the municipality of Catano, near San Juan, were still burning, police said Saturday. Gov. Luis Fortuno had given the same tally at a Friday evening news conference.
Several hundred residents were evacuated Friday and spent the night in nearby shelters. Officials raised the possibility that more could be evacuated Saturday because of the shifting wind.
Fortuno met with top officials Saturday morning and then toured some of the shelters, media reports said.
The cause of the explosion remained unknown, but officials said they will investigate. The probe will include agents from the Federal Bureau of Investigation and the Bureau of Alcohol, Tobacco, Firearms and Explosives. Federal agencies have jurisdiction over Puerto Rico, which is a territory of the United States.
Many residents said they had never witnessed anything like the fire -- at least in person.
For Carlos Salgado, the massive clouds of black smoke reminded him of the horrifying TV images he'd seen after the terrorist attacks of September 11, 2001.
Early Saturday morning, Salgado stood on the balcony of his San Juan home, about 10 miles from the blast site, and watched fireballs illuminate the night sky.
"I've never seen anything like this in Puerto Rico," said Salgado, the legal counsel for the emergency corps that manages ambulances on the Caribbean island.
He could hear explosions, and acrid air burned his nostrils as winds shifted westward, carrying the toxic smoke over populated areas of the island. The wind had been blowing the smoke to the north Friday, over less populated areas and the Atlantic Ocean.
The blaze erupted shortly after midnight Friday, when at least one fuel tank exploded. Residents described a surreal scene.
"I was in bed and all of a sudden heard this really horrible sound, so I ran upstairs and thought the whole town had blown up," said Teo Freytes, who filed an iReport for CNN.
Others woke up Friday morning to an extremely unusual sight.
"I didn't expect to see a mushroom cloud from my house," said Justin Gehrke, a U.S. Army civilian employee who also filed an iReport.
Fortuno said he declared a state of emergency for the area so Puerto Rico can get aid from the Federal Emergency Management Agency.
The storage and refinery complex sits near San Juan's bay, and Fortuno said officials were working to protect the water.
"We have serious worries that the bay or other bodies of water could be contaminated," he said.
Caribbean Petroleum's Web site says the complex has storage facilities for gasoline and gasoline-related products.
"We have been monitoring the water visually and we have installed preventive pads and other material to contain a spill," said Pedro Nieves, chairman of the Puerto Rico Environmental Quality Board. "No oil has reached the water."
But Nieves said Caribbean Petroleum has a history of spills and "there was potential that it had contaminated ground water."
Officials also are concerned about the smoke, which can be toxic for people with breathing conditions.
"This is a tremendous amount of smoke, and fire contains all kinds of irritants and this is oil that is burning," said Mary Mears, spokeswoman for the Environmental Protection Agency region that includes Puerto Rico. "It's smoke, so you're going to notice coughing, tearing, maybe a sore throat."
The air quality index Saturday morning indicated little smoke contamination, the Environmental Quality Board said in a bulletin.
The ATF sent a team of fire investigators, but have yet to determine whether the fire was an accident or intentionally set, said Orlando Felix, resident agent in charge in Puerto Rico.
FBI Special Agent Harry Rodriguez confirmed the agency is investigating graffiti found on two San Juan tunnels that referred to a fire. A spray-painted message on the two tunnels, less than three miles apart, said: "Boom, fire, RIP, Gulf, Soul, ACNF."
Caribbean Petroleum owns the Gulf Oil brand in Puerto Rico.
Rodriguez and San Juan police spokeswoman Maria Del Pilar Bon said they did not know what or who ACNF is.
Felix said investigators are not discounting any possibilities. The governor echoed that sentiment.
"There could be many reasons," Fortuno said. "We're not going to guess. But there has to be an investigation."
Caribbean Petroleum spokeswoman Frances Rios said five employees were at the facility at the time of the explosion but declined to provide more information, El Nuevo Dia newspaper reported on its Web site.
Caribbean Petroleum, which owns 200 gas stations in Puerto Rico and several inland distribution facilities, supplies much of the island's fuel. But the governor said Puerto Rico has enough gas to last 24 days and 20 days' worth of diesel.
Department of Consumer Affairs Secretary Luis Rivera Marin said 16 million gallons of gasoline were in transit to the island. He also froze prices at the level they were at 8:06 a.m.
Officials transferred 295 inmates from a high-security prison in the area to other facilities, which they declined to identify for security purposes. Another 1,600 prisoners were moved from another nearby facility. Some 200 extra prison officials were brought in to handle the transfers, Fortuno said.
Schools in Bayamon, San Juan, Toa Baja and Catano were closed, as were several roads.
Flights in and out of Luis Munoz Marin International Airport in San Juan were not affected, an airport official told CNN.
-- CNN's Jackie Castillo, Khadijah Rentas, Arthur Brice, Susan Candiotti and Moni Basu contributed to this report.