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Los Angeles firefighters battled a massive blaze at a commercial building in the Pico-Robertson area of Los Angeles on Friday.

The greater alarm fire was reported around 11:32 a.m. at 9041 W. Pico Blvd., near Wetherly Drive, according to the L.A. Fire Department. Flames were initially so heavy that responding firefighters went into defensive mode and exited the roof and building due to the fear of a building collapse.

It’s unclear what type of business is housed inside the structure, which appeared to be at least three stories high. But arson investigators and a hazard assessment team say they saw items inside that made them believe it might be a THC extraction lab, said Detective Anthony Jackson of the L.A. Impact Task Force.

The Los Angeles Police Department told KTLA the fire broke out very close to a school, but students who were there were evacuated safely. Heavy, black smoke and flames were seen billowing around and near the building for the better part of two hours.

Firefighters doused the blaze with water from aerial ladders and from the ground, video from Sky5 showed.

By 1:45 p.m., firefighters had made good progress on the fire and were working to to hit remaining hot spots.

LAFD said an “elderly” couple lived in a single-family dwelling to the rear of the building, but they were not injured. There was no fire risk to the couple’s home, but first responders stayed with and sheltered them inside their home to keep them safe from smoke exposure.

Officials said one firefighter was transported to the hospital for a heat-related illness that was not life-threatening.

The cause of the blaze was not immediately known.