Catherine Kennedy, 55, was driving home from St. Timothy Catholic Church in Escondido when she was hit in the head by a stray bullet.
"I heard boom boom boom boom, and I came running across the street to her. I checked her pulse," witness Barbara Allen tearfully told KFMB-TV. "She was gurgling and she was still alive. Then, then ambulance came and that was it."
In a news conference, Escondid police chief Craig Carter pledged to find Kennedy's killer.
"I'm extremely angry. This is a senseless death. This should not have happened," he said.
Police have already identified an apartment complex where they believe the violence originated. Investigators could be seen gathering evidence into Wednesday.