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The solar panel-covered roof of a 500,000 sq-ft cold storage facility in Los Angeles caught fire Wednesday, sending up thick clouds of dark smoke and ammonia, leading city officials to order nearby residents to shelter in place.

About 110 firefighters responded in the afternoon to a blaze at the cold storage and freezing warehouse owned by Lineage in the Boyle Heights neighborhood near downtown, according to spokesperson Jennifer Middleton of the Los Angeles fire department. Both the city and Los Angeles county deployed firefighters and hazmat teams to contain the fire.

The cause of the fire remains under investigation, Middleton said. By the time firefighters arrived, the solar panels covering the roof had caught fire.

“I urge everyone in the impacted area to get indoors immediately, close windows and doors, turn off air conditioning, and avoid unnecessary travel to the area,” Los Angeles mayor Karen Bass wrote in a tweet.

Firefighters first tried to stop the fire by spraying it with hand lines on the roof, but were forced to retreat after an ammonia line burst, according to LAFD chief Jaime Moore.

“At some point, an ammonia line was compromised and we had a large, pressurized off-gassing of ammonia,” Moore told NBC News 4.

Cold storage facilities rely on ammonia as a cost-effective refrigerant, but it is both toxic and flammable. Moore warned residents with health conditions to avoid breathing the contaminated air by staying indoors with windows closed and air conditioning off while the city monitored the air for evidence of ammonia off-gassing.

The fire grew so large and hazardous to approach that responders took the unusual step of dousing it from above with helicopters, while firefighters retreated from the roof. At least three helicopters delivered more than half a dozen air drops of water, according to Middleton.

“The amount of fire that they had was overrunning the amount of water we were able to put on it and our firefighters had to evacuate the area for a few minutes there while it was off-gassing,” Moore said.

Lineage did not immediately respond to a request for comment.

The roof fire had been largely contained by the evening, Middleton said. But the roof showed signs of sagging and the presence of solar panels raised concerns the building may contain lithium ion batteries to store the energy gathered from the sun. Moore called lithium ion batteries “one of our newest challenges in the fire service”.

Lithium ion batteries are capable of a process called “thermal runaway” that makes it difficult to extinguish them.

“It’s extremely hard to fight the fire, it’s dangerous to be around the fire, and it’s dangerous to be around the extremely hazardous smoke,” Middleton said of lithium ion battery fires.