The shooting happened Monday around 9:15 a.m. in the parking lot of a shopping center near Margarita and Murrieta Hot Springs roads.
The man wounded was hospitalized in critical condition after he was shot multiple times, police said.
"It appears that the off duty-officer had witnessed a road-rage incident that he was not involved in, and shortly thereafter is when the officer-involved shooting occurred," Murrieta police Lt. Miguel Garcia said. "Officers did locate a firearm at the scene, and the officer is cooperating with the investigation."
Marguerite Walters described hearing multiple gunshots. Her son recorded video as she walked up to offer help to the wounded man.
"I assessed him, got his name," Walters said. "He wasn't able to talk."
Footage appears to show the wounded man on the ground next to a car as the off-duty police officer keeps his weapon drawn. A woman is heard frantically screaming in the video.
Murrieta police said the firearm recovered was a replica and belonged to the man who was shot.
Details about the road-rage incident are unclear. It's also unknown what prompted the off-duty officer to open fire.
Witnesses told Eyewitness News that the off-duty officer intervened when he walked out of a gym and noticed two other men arguing.
Based on witness statements, it appears the off-duty officer said the man pointed the replica firearm at someone.
"When we first stopped to see what was going on, I heard him say, 'You can't go around pointing guns at people,'" Walters said. "But at that point the gunshots had already gone off."
A multi-agency investigation is underway.