MEXICO CITY (AP) — Prosecutors in northern Mexico said Thursday that criminals apparently killed a mother to punish her for the years she spent years searching for her missing daughter, a hunt that led to the arrest and jailing of suspects.

Miriam Rodriguez was killed in the northern state of Tamaulipas on May 10, which is Mothers Day in Mexico. She was one of many Mexicans who have taken it upon themselves to search for missing loved ones because police can't or won't.

Irving Barrios, the Tamaulipas state attorney general, said that two men had been arrested and charged in the slaying and that prosecutors believe they killed Rodriguez in revenge for her investigations.

"The strongest line of investigation is that this was an act of revenge for the work that Miriam carried out and which allowed for the detention of several people in relation to her daughter's kidnapping," Barrios said.

Barrios said the men knew at least one other suspect who had been jailed for the kidnapping and killing of Rodriguez' daughter Karen in 2014.

Four other suspects have been implicated in the killing of Rodriguez, who belonged to the group Citizen Community in Search of the Disappeared in Tamaulipas.

The group said Rodriguez's tireless search had eventually led to the discovery of her daughter's remains in an unmarked grave and she identified to authorities the man she held responsible.

Such groups have spread across Mexico in recent years as people search for their loved ones in the face of government inaction or indifference. Some 30,000 people have disappeared in the decade since Mexico announced a war on drug cartels.

Tamaulipas, which borders Texas, has been plagued by drug violence, massacres and disappearances.