A Mexican Workers' Party activist was shot dead in the western Michoacan state shortly before polls opened for general elections, local law enforcement officials said.
Flora Resendiz Gonzalez died "after she was shot at 6:30 am (1130 GMT) yesterday while she was at home" in the town of Contepec, the officials added.
The murder darkened what is already considered to be the most violent electoral campaign in Mexican history, with at least 145 politicians killed since September, according to the consulting firm Etellekt.
Shortly before news of the killing, the head of Mexico's INE electoral institute Lorenzo Cordova had praised a "peaceful election day, with no major incidents."