(CNN) -- Five tanks were still burning Saturday evening at a fuel storage complex in Puerto Rico, another 12 were still smoking but posed no danger and four had burned themselves out and collapsed, Gov. Luis Fortuno said.
Hundreds of firefighters have been battling the blaze at the Caribbean Petroleum Corp. facility in the municipality of Bayamon, near San Juan, since shortly after midnight Friday. The initial explosion shook the ground with the force of a 2.8 magnitude earthquake.
"Our mission right now is to contain the fire and not have it affect the other tanks," Fortuno said at a news conference.
President Barack Obama declared Saturday that a emergency exists in Puerto Rico, freeing up federal aid.
Firefighters will likely remain busy.
"It's quite an intense fire, and it's going to burn for quite a while," said Jeff Wanko, the lead investigator with the U.S. Chemical Safety Board, an independent government agency that investigates industrial chemical accidents.
Eric Guzman, vice president of marketing for Caribbean Petroleum, said the company hired a firm that specializes in putting out fires, El Nuevo Dia newspaper reported. The private firefighters were expected to arrive Saturday night. Guzman added that he could not say how much fuel was in the tanks when the fire started, but the blaze could be extinguished by Monday afternoon.
The cause of the explosion remained unknown but officials were investigating. Agents from the Federal Bureau of Investigation and the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the storage facility Saturday and "treating it as a crime scene," Wanko said.
The crime scene designation is routine, he said, and investigators rarely find criminal intent.
Federal agencies have jurisdiction over Puerto Rico because it is a territory of the United States.
Air quality remained good despite the fire's thick plume of black smoke, the governor said. The Environmental Protection Agency will continue to monitor the air, he said.
"We're not finding levels that would be of concern," said EPA spokeswoman Bonnie Bellow. "The fire is so intense that it is burning off chemicals that are part of the fuel."
The Caribbean Petroleum storage complex sits near San Juan's bay, but no water contamination had been found other than a small oil slick at a nearby channel that had been contained, Fortuno said.
A containment pool on the property has caught the runoff from the water used to fight the fires, Fortuno said. That water will be removed and treated, he said.
Officials had been concerned because a wind shift Saturday had put more residents at risk from smoke inhalation, but there were no reports of any problems.
Authorities also had been concerned that precipitation Saturday mixed with the smoke could lead to acid rain, but that also had not presented any reported problems.
The governor sought to allay any fears over gasoline supplies. Caribbean Petroleum, which owns 200 gas stations in Puerto Rico and several inland distribution facilities, supplies much of the island's fuel.
In the next 48 hours, Fortuno said, 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.
"There's no reason to worry in the least," he said.
Even though hundreds of nearby residents had been evacuated, Fortuno said "there is no need to move anyone else at this moment." More than 500 people spent Friday night in two shelters.
About 1,450 inmates were transferred from prisons near the fire and taken to other facilities throughout the island.
There were other disruptions: Boy band Jonas Brothers rescheduled until December a concert that was supposed to be held Saturday in San Juan.
Many residents said this was the first time they had encountered 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 thick 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.
Fortuno said he declared a state of emergency for the area so Puerto Rico can get aid from the Federal Emergency Management Agency. The fire, he said Saturday, had already cost the island $6.4 million. Money can be recovered, he said, from the oil company, FEMA and the EPA.
FBI Special Agent Harry Rodriguez confirmed Friday that 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 said he did not know what or who ACNF is.
Investigators are not discounting any possibilities, said Orlando Felix, resident ATF agent in charge in Puerto Rico. 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, said Bellow, the EPA spokeswoman. Caribbean Petroleum, she said, is under a corrective-action plan.
Pedro Nieves, chairman of the Puerto Rico Environmental Quality Board, said Caribbean Petroleum has a history of spills. Fortuno, the governor, announced Saturday that some aspects of life on the island will soon get back to normal. All but one elementary school in Catano will be open Monday. Schools in Bayamon, San Juan, Toa Baja and Catano had been closed Friday.
CNN's Moni Basu, Ross Levitt and Khadijah Rentas contributed to this report.