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.