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.

In March of this year, one of the gunmen jailed for the murder of Karen Alejandra escaped prison. Martinez began to receive death threats shortly thereafter.

Colleagues of Ms. Martinez have said she sought protection from the police to no avail. She once chased down other hitmen from the Zetas cartel after they attempted to kidnap her husband, alerting the Mexican army in the process, who eventually arrested the would-be kidnappers.

Following her murder this week, state authorities pushed back on claims they failed to protect Ms. Martinez, saying her home was patrolled and checked three times a day by police.

Martinez’s efforts to spur a local response to the kidnappings was part of a wider trend across Mexico. Most notably in October 2014, when 43 students disappeared. A government report concluded that the group was arrested and handed over to a local gang to be murdered by corrupt municipal police, although forensic evidence is scant and could not corroborate the official story. The incident prompted citizens to take investigatory work into their own hands.