The fire was reported at about 5:30 a.m. at Cheque Tires in the 1200 block of Santa Anita Avenue, Los Angeles County firefighters said. More than 50 firefighters battled the flames for an hour.
At least two men were trapped on the property, but only one escaped.
"He was on fire. The chain-link fence that was locked, that he couldn't scale, and then there was a block wall that had razor wire," Los Angeles Sheriff's Department Det. Victor Lewandowski said.
A 12-year veteran firefighter attempted to saw the fence, and suffered second-degree burns to his face and was taken to a hospital in stable condition.
Detectives initially reported that one body was found, but later found two additional victims inside the shop. It is unclear if the two bodies were the men who were trapped on the property.
"Unfortunately, I was unsuccessful in trying to help another family. A father is unable to go home to his kids," said the firefighter, who wanted only to be identified as Michael.
A dog was also on the property and was rescued.
Witness Isaac Lopez said he saw three men throw a Molotov cocktails into the business and run from the scene. Detectives said the three suspects drove away in a white Chevrolet Silverado pick-up truck.
Arson investigators were looking into that claim and the cause of the fire. The investigation is ongoing.
Anyone with information is asked to contact the Los Angeles County Sheriff's Department's Homicide Bureau at (323) 890-5500. Those wishing to remain anonymous may call "Crime Stoppers" at (800) 222-8477.