Engine 9 was the first truck to respond to a call on Saturday night about a fire in downtown Los Angeles.

Firefighters entered the one-story commercial building and climbed to the roof — a standard procedure to extinguish a blaze. But soon they heard a rumbling, high-pitched sound — as if jet engines were rearing up.

Within moments, the warehouse erupted and a 30-foot-wide fireball forced the responders to retreat. The explosion rocked the area and set multiple buildings ablaze. About 230 other firefighters ultimately reached the scene to extinguish the inferno, which took about two hours.

The debris from the building that exploded included the remnants of Engine 9: a charred firetruck, melted helmets and empty fire extinguishers that firefighters probably used to douse their burning comrades.

“This quickly turned into an incident that we all dread,” said Capt. Erik Scott, a spokesman for the Los Angeles Fire Department, adding that it was “one of the worst incidents in recent history that we’ve been through.”

“We had firefighters driven off the roof, frantically scurrying down the aerial ladder to safety through a blowtorch,” he told The Washington Post, recalling the night.

A dozen firefighters were rushed to the Los Angeles County-University of Southern California Medical Center with burns ranging from minor to severe. They are expected to survive, LAFD Medical Director Marc Eckstein said at a news conference Saturday night outside the hospital. Four were taken to the burn unit. Eight remained hospitalized Sunday.

Eckstein said that the burns were mostly on the firefighters’ arms, but that some were on their backs, ears and hands. Two firefighters were put on ventilators to treat chemical inhalation.

“Things could have been so much worse today,” he said, adding that there’s no evidence that the blast caused the injuries. Fire Chief Ralph M. Terrazas said at a news conference that the firefighters knew to flee from the sound of pressure building.

Arson investigators are looking into what sparked the blaze in the building, which is on Boyd Street in the city’s Toy District.

The building housed the wholesale business Smoke Tokes, which Scott says makes “butane honey oil.” Butane, an odorless gas that easily ignites, is used in the process that creates hash oil with THC from cannabis. Firefighters arriving at the scene saw canisters of butane on the street outside the building.

Hash oil is used in vaporizers, waxes and other products that the Smoke Tokes business retails on its website. Smoke Tokes didn’t immediately respond to a request for comment.

The commercial sale, distribution and production of cannabis is legal in California.

Amid the coronavirus pandemic, public officials have discouraged the public from gathering at the scene, which is cordoned off within a three-block radius, Scott said. Firefighters are also restricted from visiting their injured comrades at the hospital.

Los Angeles Mayor Eric Garcetti expressed gratitude at the Saturday news briefing to the firefighters and hospital workers who have responded to the blaze and the coronavirus outbreak.

“We’ve been saying for the last two months, even more than usual, how much we appreciate our medical personnel and first responders,” he said, “and, tonight, I’m doubly and deeply grateful for both of them.”