(CNN) - A large fire broke out early Friday at an oil refinery in Philadelphia, rattling the area with at least one booming explosion and lighting up the dark sky with huge balls of gas and flame.
The explosion and fire came from a vat of butane -- one of the products that come from the process of refining crude oil -- at the Philadelphia Energy Solutions refinery starting around 4 a.m., Deputy Fire Commissioner Craig Murphy said.
Murphy confirmed during an afternoon press conference that propane was also in the vat.
The refinery's fire crews have been fighting the flames with city fire crews in support, and the blaze has been contained to the general vat area, Murphy said.
During the press conference, Murphy said the fire was not under control, but was "confined and contained." The fire cannot be placed under control because of the propane and butane that's burning in the vat, which is being fed by a main. Murphy said the main has to be shutdown to put out the fire, but Philadelphia Energy Solutions says they are unable to get to the valve that would shut off the main.
This creates a dilemma for the fire department, Murphy said, because if firefighters were to put out the fire, "whatever's blowing out of the main blows right into the atmosphere."
"It is safer if it burns itself out," Murphy said.
By 7 a.m., the flames were significantly lower, video from CNN affiliate WPVI showed -- a stark contrast to the massive fireball that bystanders recorded on social media earlier in the morning.
Emergency medical services treated one individual on the scene, according to Fire Commissioner Adam Thiel. Four employees also suffered minor injuries, and all were treated on-site by a company medical team, according to Philadelphia Energy Solutions.
Details about what started the fire weren't immediately available. Authorities are working to account for all the personnel, Corley said -- though Murphy said no one who'd been at the facility is missing.
Employees "were working somewhere close to the vat" when the fire started, "but far enough away that the initial explosion didn't injure anyone," Murphy said. He said he didn't know what work was being done.
Residents reported several explosions that were felt in South Jersey and Delaware County, Pennsylvania, according to WPVI.
Small fires were burning near the vat as well, Murphy said. The extent of damage to the refinery wasn't clear early Friday, but it appeared to be confined to the vat area. Tanks surrounding the area are being cooled as a precaution, he said.
Emergency management officials initially asked people east of the fire to shelter in place, but by 7 a.m., that order was being lifted, Murphy said.
The city's Department of Public Health Air Management Services Lab "is testing samples taken from up- and downwind of the refinery fire," according to James Garrow, spokesman for the department.
The department will continue working with Philadelphia Energy Solutions to monitor air quality, Garrow said, before adding that he was "not aware of any immediate danger from the fire."
"Preliminary testing both at the site of the refinery and in the adjacent community has shown no ambient carbon monoxide, hydrocarbons (combustibles), or hydrogen sulfide," said Garrow. Additional test results are expected late Friday.
Portions of Interstate 76 and the city's Platt Bridge were temporarily closed after the fire, according to the city's Office of Emergency Management.
It was the second fire to occur at Philadelphia Energy Solutions this month. The refinery experienced another blaze on June 10, Philadelphia Fire Department spokeswoman Kathy Matheson told CNN. No injuries were reported in the earlier fire, according to WPVI.
Philadelphia Mayor Jim Kenney said the two fires were "unrelated in their nature and cause." Kenney said he has asked company and city officials "to convene a working group" to "explore concerns" regarding the two fires.
The facility -- in the south of the city, a few miles from Philadelphia International Airport -- is the 10th largest refinery in the United States and the largest oil refining complex on the Eastern Seaboard, according to its website. Philadelphia Energy Solutions employs more than 1,000 people and refines 335,000 barrels of crude oil a day.
CNN's Melissa Alonso contributed to this report.
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