MEXICO CITY — A Mexican woman who became an activist for parents of missing children after her daughter was kidnapped and murdered five years ago was fatally shot by gunmen who broke into her home this week, the authorities said Thursday.

The woman, Miriam Elizabeth Rodríguez Martínez, who lived in the municipality of San Fernando, was killed late Wednesday — Mother’s Day in Mexico. She died on the way to a hospital.

San Fernando is in the state of Tamaulipas, which runs along the Gulf Coast up to the jagged southeastern border of Texas. It is among the most violent regions of Mexico. According to government figures, it has the highest number of disappeared people in the country. So many journalists have been threatened into silence by gangs there that the stranglehold of organized crime goes largely unreported.

Karen Alejandra Salinas Rodríguez, Ms. Rodríguez’s daughter, vanished in 2012. Ms. Rodríguez began a search that lasted two years, until her daughter’s body was discovered in an unmarked grave. Afterward, she told the authorities who she believed was responsible for the murder.