LOS ANGELES (AP) — Police and fire investigators launched a criminal probe Sunday into the cause of an explosion at a hash oil manufacturer in downtown Los Angeles that sent firefighters running for their lives.

Detectives from the Los Angeles Police Department’s major crimes division were working with the city Fire Department’s arson investigators to determine what might have sparked the blast that shot a ball of flames out of the building Saturday night and scorched a fire truck across the street, police spokesman Josh Rubenstein said.

“We’re in the very early stages of the investigation … to understand what happened and figure out how to move forward,” he said.

The Federal Bureau of Alcohol, Tobacco, Firearms and Explosives was assisting local fire investigators, an agency spokeswoman said.

The blast injured a dozen firefighters including some who ran out onto sidewalks, where they tore off their burning protective equipment including melted helmets, officials said.

“The was one of the worst scenes I’ve seen,” said Los Angeles Fire Department Capt. Erik Scott.

Firefighters first thought they were battling a routine structure fire, Scott told KNX Radio, but as they got a little farther in the building they started to hear “a loud hissing sound and a significant rumbling that you could feel vibrating throughout the area.”

He said “one significant explosion” shook the neighborhood around 6:30 p.m. Firefighters inside had to run through a wall of flames he estimated as 30 feet (9 meters) high and wide, and those on the roof scrambled down a ladder that was engulfed in flames.

Three firefighters were released after spending the night in the hospital, fire department spokesman Nicholas Prange said Sunday. Of the eight that remained hospitalized, two were in critical but stable condition, he said. Officials initially announced that 11 firefighters were injured but Prange said a 12th was treated and released for a minor injury.

All were expected to survive.

“The good news is everybody’s going to make it,” Mayor Eric Garcetti said at a late Saturday news conference outside the Los Angeles County-University of Southern California Medical Center where all the injured were treated.

“Things could have been so much worse,” said Los Angeles Fire Department Medical Director Dr. Marc Eckstein, who works at the hospital and helped treat the injured.

There was light to moderate smoke when firefighters entered the one-story building in the city’s Toy District and went on the roof — normal procedures to try to quickly knock down any flames.

Los Angeles Fire Chief Ralph Terrazas said one of the firefighters inside the building thought things didn’t seem right — the pressure from the smoke and heat coming from the rear of the building were increasing. He directed everyone to get out, and they quickly started exiting the building as it was rocked by the explosion.

Firefighters on the roof scrambled down ladders with their protective coats on fire. The wall of flames shot out the building and burned seats inside a fire truck across the street.

More than 200 firefighters rushed to the scene, and dozens of engines, trucks and rescue vehicles clogged the streets. The fire spread to several nearby buildings, but firefighters were able to douse it in about an hour.

The injured firefighters were rushed to the hospital. Those who remained at the scene, unaware of the seriousness of their colleagues’ injuries, were traumatized by what had transpired, Terrazas said.

“You can imagine the amount of emotional stress,” he said.

Scott said the building was a warehouse for SmokeTokes, which he described as a supplier for makers of “butane honey oil.” 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.

A call to SmokeTokes went unanswered on Sunday 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.”

Prange, the LAFD spokesman, said carbon dioxide and butane canisters were found inside the building but that it was still not clear what caused the blast.

Adam Spiker, executive director of the cannabis industry group Southern California Coalition, said he didn’t know what activities were taking place inside the building. However, if the business was using butane in cannabis extraction it would be illegal, since the city has never issued a license for that type of operation.

Because of safety concerns, such businesses are typically restricted to industrial areas and kept away from urban centers.

“If they were doing volatile extraction with butane … they couldn’t be legal in the city of LA to do those types of activities,” Spiker said.

He said the coalition was unaware of the business having any type of license and “something about this doesn’t pass the smell test.”

Information so far “puts up a lot of alarm bells,” Spiker said.

In 2016, there was another major fire at a business called Smoke Tokes at a nearby address. The Los Angeles Times reported at the time that it took more than 160 firefighters to put out the blaze and that they encountered pressurized gas cylinders that exploded in the fire.

No one was injured in the fire and it was unclear whether that business and the one that burned Saturday were connected.

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Associated Press writers Michael Blood and Christopher Weber in Los Angeles and Daisy Nguyen in San Francisco contributed to this report.