PHILADELPHIA — More than a day after a series of explosions and a spectacular fire seriously damaged a South Philadelphia oil refinery, a “very small fire” still flickered at the site early Saturday, and unanswered questions smoldered over the lasting impact of the accident.

The accident at Philadelphia Energy Solutions, the East Coast’s largest refinery, rattled windows, injured five workers, unnerved the city, and caused gasoline markets to spike on speculation of fuel shortages. The injured workers were treated at the scene.

Emergency responders from the refinery and the Philadelphia Fire Department remained on the scene Saturday, PES said in a statement. The company said it had not made a decision to let the fire burn itself out, but the fire may need to consume any remaining combustible material before workers can isolate a fuel line in a damaged unit that is feeding the flames.

“We continue to work to isolate the remaining line; access is limited due to the damage and instability of the remaining structure,” the company said.

Safety advocates said the danger could have been far worse had the blast released a cloud of hydrogen fluoride, a deadly chemical used in the refining process.

The U.S. Chemical Safety and Hazard Investigation Board, an independent federal agency, is dispatching a four-person team to Philadelphia to investigate the incident, which was similar to refinery accidents in Torrance, Calif., in 2015 and Superior, Wis., last year that came perilously close to releasing hydrogen fluoride. The Wisconsin accident prompted a temporary evacuation of residents.

“Philadelphia and surrounding communities appear to have narrowly dodged a catastrophe this morning,” Joseph Otis Minott, executive director of the Clean Air Council, said in a letter calling for the CSB to investigate.

PES said three explosions “impacted” a unit that produces alkylate, used to boost gasoline octane. The refinery complex has two alkylation units, and the unit that was involved in the conflagration uses hydrogen fluoride – also known as hydrofluoric acid – as a catalyst. A 2009 release of just 22 pounds of the chemical at the South Philadelphia refinery sent 13 contract workers to the hospital.

“Hydrogen fluoride is extraordinarily dangerous,” said Fred Millar, an Arlington, Va.-based chemical disaster expert and independent consultant. City officials said none of the material was released in Friday’s event.

The incident, about 4 a.m. Friday, lit up the predawn sky with a fireball so large that it was captured by a weather satellite. City health department spokesperson James Garrow said the city took air quality samples both upwind and downwind of the refinery. The samples were taken to the city’s Air Management Services Laboratory and were tested for 61 chemical compounds, none of which was found to be at “or even near” harmful levels.

PES refining complex firefighters responded initially and were supported by city firefighters, said Deputy Fire Commissioner Craig Murphy. The fire quickly escalated to a third alarm.

Murphy said the firefighters contained the fire to the alkylation unit, and cooled surrounding pipes and tanks with water. He said 120 Philadelphia firefighters and 51 pieces of equipment were involved in the operation.

The cause of the explosions is not known. The Fire Marshal’s Office will investigate once the scene is safe to enter, Fire Commissioner Adam Thiel said.

The fire originated in a propane tank, said Cosmo Servidio, mid-Atlantic head of the U.S. Environmental Protection Agency, adding that “no other tanks were affected.”

“We have not determined the product that was burning, but we believe it was mostly propane,” PES said in a statement. Earlier in the day, Murphy said butane was thought to be feeding the fire.

Officials lifted a shelter-in-place request for residents in the immediate area and reopened the George C. Platt Memorial Bridge, which was closed after the explosions.

“Those who live and work in close proximity to the refinery and all Philadelphians have our word – we are firmly committed to ensuring the safe operation of the refinery and the safety of those in its vicinity,” Mayor Jim Kenney said in a statement.

Wholesale gasoline prices surged 3.7% Friday in New York on speculation that the PES outage might curtail regional fuel supplies.

But Tom Kloza, global head of energy analysis for the Oil Price Information Service, said the uptick was unlikely to last long, as Northeastern fuel markets are supplied from multiple sources, including refineries on the Gulf Coast, and in Canada and Europe. “The North Atlantic market has plenty of gas,” he said.

PES can process 335,000 barrels of crude oil a day, or about 14 million gallons. It employs about 1,000 people.

The company said the refining complex was running at a reduced rate.

“We are thankful that no one was seriously injured during today’s event,” PES said in a statement, adding that its immediate focus was to continue to protect the area and assess the balance of the facility.

The refinery emerged from bankruptcy last year but has continued to struggle under a heavy debt and intense competitive pressures. Its management team has been reshuffled in recent months.

Refineries convert crude oil to a range of petroleum products, including gasoline, diesel fuel, and heating oil, through a number of process units that operate under high pressure and heat. They are inherently hazardous.

Kenney said in a statement that he spoke Friday with PES leadership, as well as the Fire Department and Managing Director’s Office, and “was assured that the two incidents are unrelated in their nature and cause,” and was informed of the speed with which notifications went to nearby residents.

“Still, I believe that there is room for improvement, both in the operation of the refinery in light of two fires in as many weeks, and in the communication to residents,” he said.

The EPA worked with the state and city to set up “fence line” air monitors “along the perimeter” of the refinery and possibly in the community to keep a check on air quality, Servidio said.

Monitoring produced normal readings for explosives, carbon monoxide, oxygen, and hydrogen sulfide, a dangerous gas, the company said.

“The health department has no findings that would point to any immediate danger in the surrounding community at this time, and the city is not recommending evacuation or shelter-in-place,” Garrow said in a statement.

David Masur, executive director of PennEnvironment, a nonprofit environmental group, said he was concerned about any pollutants or toxins and their impacts on nearby neighborhoods.

“If I live about a mile away, and could hear it in the dead of night, you can imagine what it’s like if you lived next door and the impact on local families,” he said. “And certainly you have to think about the first responders. They have to show up and do their job. They don’t know what to expect or what they’ll be exposed to.”

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