The animals were shot while in their pasture at Marianne Rountree's Collier Canyon ranch.
The reward for information about the shooting now stands at $17,000, following a contribution from the East Bay SPCA.
Investigators are looking for the driver of a white Honda that crashed into a brabed wire fence in the area the same evening.
Anyone with information about the shootings is urged to call the Contra Costa Sheriff's Office at (925) 646-2441.
