Updated 5:40 p.m. 

A massive explosion rocked the oil refinery in South Philadelphia early Friday morning, city officials confirmed. The U.S. Chemical Safety and Hazard Investigation Board is opening an investigation into the incident.

Fires raged throughout the morning at the 150-year-old industrial complex at 3100 W. Passyunk Ave. The refinery is located along the Schuylkill River, just south of Girard Estates and next to FDR Park.

As of 4:30 p.m., the fire was “confined and contained,” but not yet under control. The city’s Office of Emergency Management is working with the Fire Department, Philadelphia Gas Works, PECO and the Coast Guard to coordinate with refinery owner Philadelphia Energy Solutions on the response to the disaster.

PES said it recorded four minor injuries to workers, all of whom were treated on site by company medical staff. The cause of the blaze is still unknown. “It’s gonna be a pretty long and drawn out process,” said Deputy Fire Commissioner Craig Murphy at a Friday afternoon press conference.

Nearby residents heard and felt explosions and saw flames shooting into the sky, turning night into day.

“It looked like the sun was coming up early,” said Damon Hudgens, 26, who works at an airport parking lot. “But you looked to your left it was just a big ball of fire. It just kept rising up and sprawling out.”

No evacuation orders were issued. There was an early shelter-in-place order for residents east of the refinery, but it was lifted around 7 a.m.

Here’s what we know about the blast and its aftermath.

What we know

• There were three separate explosions. which happened at approximately 4 a.m., PES said in a statement.
• The structure impacted is what’s known as an “alkylation unit,” per the statement.
• Deputy Fire Commissioner Craig Murphy originally said it was a butane vat that exploded. PES later said it believed propane was the main fuel that caught fire. At a Friday afternoon press conference, Murphy confirmed that “a mix of propane and butane” were continuing to feed the flames.
• Refinery owner Philadelphia Energy Solutions has its own fire brigade, and when Fire Dept. responders arrived on scene around 4:45 a.m., they were already beginning work to isolate the fire.
• The blaze escalated to 3 alarms, and the Fire Dept. deployed 51 apparatus and 120 responders to the scene.
• PFD cannot put out the fire without the OK from PES, Murphy said on Friday afternoon, explaining that extinguishing without the right precautions carried the risk that whatever is inside the burning tank would “blow right into the atmosphere.”
• Four minor injuries to PES workers on site are the only injuries reported so far.
• Homes as far away at South Jersey were shaken by the boom, according to NBC10.
• Multiple SEPTA bus routes have been diverted because of the fire. The Schuylkill Expressway was briefly shut down, but has reopened.
• The Passyunk Avenue Bridge and Penrose Avenue Bridge were also originally shut down; the former reopened at 6 a.m. and the latter at 7:10 a.m., PES said.
• The refining complex is still operating, but it’s running at a reduced rate, per PES.

What we don’t know

• What caused the fire. It’s the second fire at the refinery in one month, following a June 10 fire in which no injuries were reported.
• When authorities will be able to bring the blaze under control. Officials said they are deploying systems that not only fight the fire from above but also from below. “All we’re doing is supporting the efforts of PES right now,” Deputy Fire Commissioner Murphy said. PFD has extra personnel on the scene waiting to get word from PES.
• What effect the smoke will have on the air quality in Philadelphia, or how long the smoke will linger. Preliminary testing on Friday morning showed no ambient carbon monoxide, hydrocarbons (combustibles), or hydrogen sulfide, according to the Philly Health Department.

Background on the refinery

The PES refinery is one of the oldest and largest on the East Coast, and turns up to 350,000 barrels of raw crude oil into gasoline, jet fuel, heating oil, diesel, and chemicals every day. Once owned by Sunoco, the refinery was once on the verge of closure until the shale boom unleashed record-breaking amounts of cheap, domestic oil and gas.

The refinery is also Philadelphia’s single-largest source of particulate pollution.