Misak Minasyan, 60, and Hripsime Minasyan, 59, arrived at the graveyard at Bellevue Memorial Park in Ontario on Saturday to find Karapet Kalajian already there waiting for them, the Associated Press reported.
Kalajian approached the couple and a third relative as they got out of their car and began firing.
After he fatally shot the couple, Kalajian turned the weapon on himself, authorities said, calling the incident a murder-suicide.
The third relative, an elderly woman, was not harmed, the AP wrote.
Kalajian had reportedly become estranged from the family and was distraught because January 3 is the five-year anniversary of his late wife’s death.
A man who lives near the cemetery told the Ontario Daily Bulletin that he heard 20 shots before police arrived.
A motive for the killings was not immediately clear.