Investigators on Monday began combing the wreckage of a fire and explosion at a downtown Los Angeles vape and marijuana accessories wholesaler to determine its cause as 12 injured firefighters started the long process of recovery.

Seven burned firefighters have been released from the hospital — four on Monday — while four remain hospitalized and are considered to be stable, though their conditions were not immediately available. The twelfth firefighter suffered a minor injury. All are expected to survive.

Authorities have not yet determined if the business was operating legally or if it had been recently inspected. The Bureau of Alcohol, Tobacco, Firearms and Explosives has sent in special agents from around the country to help with the criminal investigation. They are expected to help reconstruct the scene, identify where the fire started and determine what caused it.

The blaze began late Saturday afternoon in a one-story warehouse for SmokeTokes, which officials have described as a supplier for makers of “butane honey oil” in the city’s Toy District. Butane is an odorless gas that easily ignites, and it’s used in the process to extract the high-inducing chemical THC from cannabis to create a highly potent concentrate also known as hash oil. The oil is used in vape pens, edibles, waxes and other products.

Carbon dioxide and butane canisters were found at the scene but that it was still not clear what caused the explosion. Los Angeles Fire Chief Ralph Terrazas told The Associated Press that while it looked like the warehouse had “a lot of stored material,” he hadn’t heard of any processing of materials or honey oil extractions taking place there.

The city’s Department of Cannabis Regulation does not have any records of a license or a commercial cannabis application for SmokeTokes, officials said Monday. A call to SmokeTokes went unanswered Monday, and the company’s voicemail was full.

On its website, SmokeTokes advertises a variety of products including “puff bars,” pipes, “dab” tools, vaporizers, “torches and butane,” and cartridges. The company says it is “an international distributor and wholesaler of smoking and vaping products, and related accessories.”

The business has been registered with the state since 2011. It is currently listed as a limited liability company that sells smoking and tobacco accessories. It was not immediately clear if the business was allowed to sell butane under a restrictive state law or city regulations.

Terrazas said he didn’t notice a diamond-shaped safety placard outside the business that would have warned firefighters of the potential hazards inside.

“I did not see it and I doubt that it did” have it, he said.

Capt. Robert Long, commanding officer of the Los Angeles Police Department’s major crimes division, said his detectives will be working with the ATF and “looking at all aspects of the business to see if all applicable safety rules were followed.”

The fire department stopped doing on-site inspections because of the coronavirus pandemic, Terrazas said. It’s not yet known if SmokeTokes would have been scheduled for an inspection during that period.

Firefighters arrived Saturday amid light to moderate smoke. They had to use an electrical saw to get into the building through its closed steel security doors that had been rolled down, Terrazas said, while others went onto the roof to try to quickly knock down any flames.

One of the firefighters noticed that the pressure from the smoke and heat coming from the rear of the building were increasing. He ordered everyone to get out but a thunderous explosion rocked the building. Firefighters on the roof scrambled down ladders through the blaze, with their protective coats aflame. Some ran out onto sidewalks, where they tore off their burning yellow gear, including melted helmets. The wall of flames shot out the building and burned seats inside a fire truck across the street.

Terrazas said he watched video footage of the blaze — in which several firefighters had to race down an aerial ladder through a fireball to safety — and was “amazed” they survived an explosion like he’s never seen before in his 36-year career.

He credited it, in part, with their protective equipment that is rated higher than the national standard. City firefighters wear yellow gear that is meant to withstand flames for 19 seconds without developing second-degree burns. The National Fire Protection Association’s standard is 17.5 seconds.

Even in their heavier equipment, 11 firefighters suffered burns from minor to serious. Two initially had to be put on ventilators.

“Without a doubt, the burns would be more severe” if they had been wearing the 17.5-second gear, Terrazas said. “I think we’re on the road to recovery.”

A GoFundMe campaign was set up to help the family of one of the more seriously injured, a captain. The money would be used to bring meals to his wife and two children since restaurants were closed at their hospital-adjacent hotel, the page’s author said.

“Money will also be raised for long term help they will need as he will not be returning to work soon,” the page states.