MEXICO CITY (AP) — Yet another mother searching for her disappeared child has been killed in Mexico, the fourth murder of a volunteer search activist in Mexico since the start of 2021.

Activists said Tuesday the victim was Esmeralda Gallardo, who led efforts to find her missing 22-year-old daughter.

The activist group “Voice of the Disappeared in Puebla” said Gallardo was killed in the city of Puebla, east of Mexico City.

Prosecutors in Puebla confirmed the killing, and pledged to solve the case “as quickly as possible.”

But the group called on authorities in a statement to “leave aside the superficial speeches and guarantee the safety of the victims, and the rights and safety of the families of the disappeared.”

The U.N. human rights office in Mexico said Gallardo had been shot to death. The U.N. office condemned the killing and said in a statement that Gallardo “had provided relevant information about her daughter’s disappearance which was not effectively taken into account during the search or the investigation of the crime.”

Gallardo’s daughter, Betzabe Alvarado Gallardo, disappeared in the low-income neighborhood of Villa Frontera in January 2021.

In August, search activist Rosario Rodríguez Barraza was killed in the northern state of Sinaloa, home to the drug cartel of the same name.

In 2021, in the northern state of Sonora, searcher Aranza Ramos was found dead a day after her search group found a still-smoking body disposal pit. Earlier that year, volunteer search activist Javier Barajas Piña was gunned down in the state of Guanajuato, Mexico’s most violent.

The motive in those killings remained unclear; in the past, many searchers have said publicly they aren’t looking for evidence to convict killers.

Most volunteer search teams are made up of mothers of Mexico’s over 100,000 missing people.

Faced with official inaction or incompetence, many mothers are forced to do their own investigations or join search teams which, often acting on tips, cross gullies and fields, sinking iron rods into the ground to detect the tell-tale stench of decomposing bodies.

The searchers, and the police who sometimes accompany them, usually focus on finding graves and identifying remains. Search groups sometimes even get anonymous tips about where bodies are buried, knowledge probably available only to the killers or their accomplices.

But the mainly female volunteers often recount getting threats and being watched — presumably by the same people who murdered their children, brothers and husbands.

After the killing in August, a group of search groups known as “collectives” issued a statement demanding protection for searching mothers.

“No mother should be killed for searching for her children,” the coalition wrote. “On the contrary, the government is obligated to ensure their safety in continuing their searches, as long as thousands of cases of disappeared people continue to pile up.”