Over lunch today, King C. Brown and I discussed the El Segundo case and I worked out this scenario for what might have happened that night. I apologize for the extremely low-tech simulation but this is all I could find around the hacienda.
1. Curtis and Phillips remove the driver from the car.
2. Curtis and Phillips shine a flashlight in the driver's face (possibly a field sobriety test).
3. Curtis gets in the car to radio the dispatcher.
4. The killer shoots Phillips in the back. Then shoots Curtis from the passenger's side.
5. As the killer flees, Phillips fires four rounds into his car.
Here's another picture of the crime scene: