The body was so badly burned that investigators have been unable to even determine the age or sex of the victim, said sheriff's Lt. John Corina.
A deputy on patrol spotted something burning on the side of East Covina Hills Road, just west of Via Verde and out of view of homes, about 3:45 a.m.
The deputy used a fire extinguisher to put out the flames, only to discover that the object was a body, Corina said.
He estimated that the body had been there for a couple of minutes before the deputy arrived.
"We're treating this as a homicide, but we won't know for sure until we get the autopsy reports," he said.
Investigators asked anyone with information on the case to call the sheriff's homicide bureau at (323) 890-5500.