Miriam Rodríguez Martínez, an activist on behalf of the thousands of Mexican citizens who go missing every year in Mexico, was murdered in her home by gunmen this week.

Martinez had been known for personally investigating the circumstances of her daughter’s death. Karen Alejandra was kidnapped and murdered by criminals from the Zetas cartel in 2012. After finding her daughter’s body in an unmarked grave, Martinez spearheaded the effort to solve the crime, ultimately giving authorities enough information to find and prosecute her daughter’s killers.

In the aftermath of her daughter’s murder, Martinez formed a network of 600 families to assist in locating their missing relatives, blaming the vacuous security situation in her home state of Tamaulipas on inept government authorities.

Gunmen broke into her home on Wednesday, May 10th—Mother’s Day in Mexico—and shot Martinez. She died on the way to the hospital.