The Los Angeles County Fire Department reported an officer collapsed and hit his head at a gun range at East Fork Road and Highway 39 in the Angeles National Forest.
The officer was identified as 23-year-old Covina resident Kevin Sandoval.
Sandoval was one of several officers taking part in firearms training at Burro Canyon shooting range in the Angeles National Forest when he collapsed for some unknown reason.
He was not firing gunshots at the time, according to authorities. They said he may have died from natural causes.
The officer was reportedly in full cardiac arrest when medical personnel arrived on scene. The officer was transported by helicopter to Foothill Presbyterian Hospital in Glendora where he was pronounced dead.
"According to physicians here at the hospital, the trauma to the face and head wasn't that severe to cause a person to cause death, so it appears he may have some kind of internal problem that caused him to fall and pass out," said Lt. John Corina of the Los Angeles County Sheriff's Department Homicide Bureau.
Sandoval was not married. He was a two-year veteran of the South Pasadena Police Department and was a cadet for a year previous to that.
"Officer Sandoval was probably what I would categorize as probably the best police officer that we would be fortunate as a city and a community to find," said Sgt. Tony Abdalla of the SPPD. "He was proactive and very community oriented and just exemplified what we look for in a police officer, and it is a huge loss."
Sandoval is survived by his parents and a sister.
The cause of death was not determined. An autopsy was planned.