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An explosion that injured 12 firefighters in downtown Los Angeles remains under investigation Sunday, but officials said it appeared to have ignited at a warehouse that had supplies to manufacture marijuana concentrate.

The Los Angeles Police Department, local fire investigators and federal officials have launched a criminal probe, law enforcement sources told the L.A. Times.

The incident happened around 6:30 p.m. Saturday near San Pedro and Third streets in Little Tokyo at a business called Smoke Tokes, where crews found small butane canisters inside and outside the one-story warehouse, LAFD Fire Chief Ralph Terrazas said.

Smoke Tokes’ website describes the company as a distributor and wholesaler of smoking and vaping products. The warehouse appeared to have supplies for manufacturers of butane honey oil, according to the Fire Department.

Firefighters who responded to the initial blaze tried to leave as smoke pressure built inside the building, Terrazas said. But an explosion occurred, injuring 12 firefighters.

The crew members were transported to the L.A. County USC Medical Center—four of them sent to a burn intensive care unit and two placed on ventilators due to inhalation of superheated gases, attending physician Dr. Marc Eckstein said Saturday night. The other firefighters suffered minor to very serious burns in their upper extremities, the doctor added.

Hospital officials expect them to “pull through,” Eckstein said.

The Fire Department previously said 11 firefighters were injured in the incident but later announced a 12th crew member who was treated for a minor extremity injury at an emergency room and released Saturday night.

Officials said three firefighters have been discharged while eight remained hospitalized, including two in critical but stable condition. Later Sunday, the two firefighters placed on ventilators were taken off them, according to LAFD Captain Erik Scott.

“They continue to remain in the intensive care unit for treatment of significant burn injuries,” LAFD said of the two firefighters.

Authorities reported no other injuries.

The incident triggered a “mayday” call and drew more than 230 firefighters who worked on extinguishing the blaze for nearly two hours, the Fire Department said.

People nearby heard the rumbling from the explosion, which sent flames and black smoke up into the air, visible from miles away.

Many posted footage of the incident on social media. One video showed firefighters coming down a ladder as flames raged near them.

Officials from Gov. Gavin Newsom to L.A. Mayor Eric Garcetti and Police Chief Michel Moore expressed concern and gratitude for the firefighters.

The next morning, crews remained on the scene as the probe continues.

“The cause is of great concern to us…” said an incident update from the Fire Department.

Scott described the explosion’s horrific aftermath: a firetruck charred from 30 to 40 feet away from the building, holes burned through firefighters’ gear and melted helmets.

“This could’ve been worse,” Scott said.