The head of a federal agency investigating last Friday’s explosions and fire at the Philadelphia Energy Solutions called the incident “catastrophic” and said the main unit where blasts occurred is still too dangerous for investigators to enter, calling it a landscape of twisted metal.

Kristen Kulinowski, interim executive for the U.S. Chemical Safety and Hazard Investigation Board (CSB), said at a briefing Thursday that a cause has not yet been determined by four investigators from the independent agency now on the scene. But, she did release new details of that day.

Events began around 4 a.m. June 21, she said, when hydrocarbon vapors were released in the alkylation unit that produces a high-octane blending agent for gasoline. The unit uses hydrofluoric acid as a catalyst. In its gaseous state — hydrogen fluoride — it poses serious health risks for anyone nearby. Officials said there was no release of HF during the fire.

“We don’t know what caused the leak yet,” Kulinowski said, only where it first occurred.

Somehow, the vapors were ignited. Kulinowski did not know the source of the ignition but said it could have been a range of things. In other refinery blasts, she said, sources of ignition have ranged from welding equipment to an electric motor.

Multiple explosions occurred during the incident, including a large blast at 4:22 a.m. The area around the blast was described to her “as a lot of twisted metal and a lot of debris that had been scattered across a large area.”

She called it a “fundamental failure in the system,” at the PES facility. “I know that this event has been very disturbing for the community surrounding the facility and the City of Philadelphia." She said employees and the community were “fortunate” there were no fatalities or serious injuries.

Kulinowski said the investigation is still in a preliminary stage, hampered by the fact that the blast was so damaging.

“I should point out that our investigators have not yet been able to enter the unit where the explosion occurred,” Kulinowski said. “The area right now is still unstable. And it’s unsafe for our investigators to be walking around out there. Our first priority will be to go in and do a physical examination.”

The agency has investigated about a dozen refinery incidents in the last 20 years, Kulinowski said, including those resulting in fatalities, serious injury, and property damage. The agency has no enforcement powers and can only make recommendations.

“Refining, you know, is a high hazard industry. Hydrocarbons are flammable, and explosive," Kulinowski said. “And that’s why the safety management systems are in place to help prevent their unintentional release. And so this is an example of where somehow some hydrocarbon got out of the pipe or the tank or whatever it was, and found an ignition source and then led to the cascade of events. ...”

Also Thursday, Philadelphia labor leader John Dougherty blasted the leadership of Philadelphia Energy Solutions after the company’s abrupt announcement that it plans to shut down the South Philadelphia refinery, and asked the company to rescind the planned closure.

In a letter to the refinery’s chief executive, Mark Smith, Dougherty, the business manager of the 50,000-member Philadelphia Building and Construction Trades Council, expressed “dismay and outrage” that Wednesday’s announcement of a closure came with no warning.

“We will use whatever government regulatory or legal means available to us should you choose not to cooperate with us,” said Dougherty, who has maintained his leadership role after being indicted in January under federal charges of embezzlement, bribery, and theft.

PES did not respond to a request for comment.

The 335,000-barrel-per-day refinery, the largest on the East Coast, sent a notice to state labor officials that it will shut the plant down Monday and lay off about 1,020 workers in the following two weeks. The fire significantly damaged equipment and systems at a complex that was already struggling financially, the company said.

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The accident is also under review by the Occupational Safety and Health Administration, the Bureau of Alcohol, Tobacco, Firearms, and Explosives, and the Fire Marshal’s Office.

The CSB, which investigates industrial fires and explosions, says that 50 of the nation’s 150 refineries operate HF alkylation units. The board in April called on the Environmental Protection Agency to revisit the effectiveness of existing regulations for hydrofluoric acid and whether there are safer technologies available for alkylation.

The CSB conducts “root cause” investigations of chemical accidents at industrial facilities but does not have the power to prosecute or fine violators. Its investigations typically take a year to 18 months to complete, but Kulinowski said Thursday the agency could release a preliminary report six to nine months from now.

The investigators may examine whether there is any connection between Friday’s accident and a “turnaround," or maintenance outage, the refinery had planned in January and February, but which was cut short, refinery workers said. The abbreviated turnaround had targeted the Girard Point side of the refinery complex, including some equipment close to the alkylation unit that was destroyed.

Kulinowski said PES is cooperating with investigators.

Staff writers Claudia Vargas and Juliana Feliciano Reyes contributed to this article.