According to the Corona Police Department, the officer was shopping with his family when the man, identified by police as 32-year-old Kenneth French, suddenly assaulted him in an unprovoked attack.
The officer, who was not identified, then fatally shot French and wounded the man's parents, Paola and Russell French, who were with him at the store. Police said it's believed the assault stemmed from an argument.
"From my understanding, from some people we talked to, there was apparently an argument inside," Corona Police Lt. Jeff Edwards told reporters.
But French's cousin, Rick Shureih, is telling a different story. He claims French had developmental problems and was nonverbal, calling him a "gentle giant."
"He's never been violent in the past," Shureih told the Los Angeles Times. "He's always been very cooperative and kept to himself."
He added: "It could have been that he bumped into somebody but couldn’t communicate the fact that he was sorry."
Shureih also urged witnesses at the store to come forward.
Paola, the man's mother, is in a coma, while Russell, his father, is expected to recover.
The officer involved has not been charged in the shooting but the incident remains under investigation.