MEXICO CITY (AP) — Rights groups are demanding justice for a missing persons activist who was gunned down at his home by unidentified assailants in Mexico's Gulf coast state of Veracruz.

Abiram Hernandez worked for years with relatives — mainly mothers — of those who have been kidnapped or disappeared. He was killed on March 29.

The Colectivo Solecito group called for an investigation into the case Tuesday and said Hernandez' family members and colleagues should receive protection. The U.N. High Commissioner for Human Rights' office has also called on authorities to punish those responsible.

Mexican cartels frequently use clandestine pits to dispose of victims.

Because of police inefficiency, groups known as "colectivos" often search for burial sites on their own.

The commissioner's office says 9 activists have been killed in Mexico in 2019.