The incident happened shortly before 10 p.m. at E-Joy Foot And Body Massage, according to Riverside Police Department.
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The customer opened fire after two men broke in and tried to rob the establishment, police said. Authorities initially identified the shooter as an employee.
The two robbers, at least one of whom was armed and dropped his gun, fled the scene. The suspect who was hurt ended up in the parking lot of a nearby Northgate Market, where he was taken into custody by police officers and transported to a hospital. The other suspect managed to get away.
A description of the suspect who remained at large was not available.
The business was closed at the time of the incident, investigators said. No employees were hurt and nothing was stolen.
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Police said the customer's gun was legally registered, and that the district attorney will investigate the shooting.
The shooter was detained and is cooperating with investigators, police said.
"Just like when you're inside your own home or a business, if harm's way comes to you, you do have a right to defend yourself if you or others are at risk of serious bodily harm or even death," Officer Ryan Railsback, spokesman for the Police Department, said.
"We tell people: Don't do it if you don't think you can do it," Railsback said. "It's best to just be a witness, to cooperate and give them what they need so you don't get hurt. It's a judgement call. It's all based on your own level of confidence on how you can handle the situation."