The shooting happened in the municipality of Angamacutiro, located in the northern part of the state.
Authorities said the victims - 50-year-old Gloria Ambriz and 53-year-old Rafael Cardona - were shot while they were sitting inside a Ford pickup truck with license plates of the state of Querétaro.
Ambriz was pronounced dead at the scene. Cardona was taken to a local hospital, where he later died.
According to the Michoacán Attorney General's Office, Angamacutiro's mayor, Hermes Pacheco, is married to Cardona's sister.
Both Cardona and Ambriz lived in the U.S. but they owned a house in Angamacutiro.
Authorities said the attack happened close to their home. Forensic experts found "ballistic evidence" at the scene that will be analyzed, authorities said.
The victim's bodies have since been handed over to the families in Angamacutiro where they will be buried.
It's unclear what part of California the couple was from.
The investigation is ongoing.
This is a developing story. This article will continue to be updated as more information becomes available.