MEXICO CITY — An Indigenous rights activist has been killed by a gunman on a motorcycle in southern Mexico, the third organizer to be killed in Mexico in the space of about one month.

The Fray Bartolomé de las Casas Human Rights Center said late Monday that Simón Pedro Pérez López was gunned down outside a market in Simojovel, a town in Chiapas state.

Pérez López, better known as Simón Pedro, was a member of the Tzeltal Indigenous group and had been active in a social justice organization born out of the 1998 massacre of 45 Indigenous people in the nearby town of Acteal.

Father Marcelo Pérez, the parish priest of Simojovel, said the area has been plagued by drug trafficking gangs, violence and injustice.

“Simón Pedro was innocent, he fought peacefully,” Pérez said. “Let’s hope this will inspire people to wake up their consciences and fight for peace.”

On Monday, authorities confirmed the murder of Yaqui Indigenous leader Tomás Rojo Valencia in the northern border state of Sonora, and over the weekend environmentalist David Díaz Valdez was killed in the Pacific coast state of Nayarit.

Prosecutors in Sonora say a criminal gang killed the Yaqui leader because they wanted money his group had raised at highway blockades.

Rojo Valencia disappeared May 27 amid tensions over months of periodic blockades that the Yaqui put up to protest gas ducts, water pipelines and railway lines that have been run across their territory without consulting them or giving them much benefit. His body was found in June, but took time to identify.

Further south in Nayarit, Díaz Valdez was shot death as he got into his car. He had led opposition to the building of a power generating plant in his largely Indigenous community on the Pacific coast, and he had recently been jailed, apparently for refusing to hand over an ambulance used by his community.

Mexico’s governmental National Human Rights Commission expressed concern over the level of killings. The commission said 12 activists have been killed so far this year, eight of whom were environmentalists.