Rocky, a 700lb (317kg) bear, bit Stephan Miller, 39, in the neck at a centre where wild animals are trained for film and TV productions.
But officials have said they are puzzled by what provoked the bear, who had been touted as one of the "best trained" in showbusiness.
Two other trainers subdued Rocky with pepper spray and were unhurt.
The attack occurred during a training session at the facility, Randy Miller's Predators in Action at Big Bear Lake, east of Los Angeles. Three experienced handlers, including Stephan Miller, were present, a spokeswoman from the San Bernardino County Sheriff's Department said.
Mr Miller was pronounced dead at the scene by paramedics.
The bear and the enclosed compound were examined after the incident by California's Fish and Game Department and no permit violations or dangers to public safety were found, agency spokesman Harry Morse said.
Authorities said that assuming no wrongdoing turns up in the investigation, it would be left up to the owners of five-year-old Rocky to decide whether or not to keep him alive.