Local and federal authorities are investigating the cause of a major fire at a building in downtown Los Angeles on May 16 in which 11 firefighters were injured when a fireball erupted.

The structure fire at 327 E. Boyd St. was reported at 6:26 p.m. The first companies arrived on scene within four minutes to find a one-story commercial building with smoke emanating from the structure. Businesses inside were not open and firefighters used saws to gain access to the structure, LAFD spokesman Nicholas Prange said.

Firefighters used hoses at the base of the building as personnel were sent to the roof to open it for ventilation. Firefighters on the roof encountered increased pressurized smoke and heard a rumbling sound indicating dangerous conditions and immediately began exiting the building. As they retreated, an explosion created a fireball that enveloped firefighters as they descended from the roof on a ladder. The heat melted helmets, burned protective coats and hoods, and charred a nearby fire truck, Prange said.

The personnel in charge sent out a mayday call and treatment began for the 11 firefighters who had been injured. They were all from Fire Station 9, located at 430 E. Seventh St., in the Skid Row area. Dozens of additional fire trucks and ambulances from the surrounding area were dispatched to the scene to assist with medical treatment and fire suppression, Prange added.

The 11 injured firefighters were taken to Los Angeles County-USC Medical Center. As of May 18, five had been released. The remaining six firefighters are in stable condition and are expected to be released in the coming days.

At the peak of the incident, more than 230 firefighters were deployed to the blaze, which was extinguished at 8:08 p.m. Los Angeles Fire Department Arson investigators are working with the Los Angeles Police Department’s Major Crimes Bureau and the federal Bureau of Alcohol, Tobacco and Firearms to investigate the cause.

The fire occurred in a space occupied by Smoke Tokes Wholesale Distributor, an alleged supplier of ingredients to make butane honey oil, a form of cannabis. It was unclear if the business was operating legally. Prange said the LAFD will release additional information about the incident in the coming days.

LAFD Chief Ralph Terrazas announced on Tuesday that the fire department will inspect all similar cannabis businesses throughout the city in the coming weeks to ensure chemicals are being stored safely and there are no other violations.