A new mayor was sworn in Thursday in a city in southern Mexico where his predecessor was killed and beheaded less than a week after taking office.

The new mayor, Gustavo Alarcón, a doctor, had been elected as an alternate on the same ticket as deceased Mayor Alejandro Arcos in the June election.

Arcos took office on Oct. 1 in the violence-wracked city of Chilpancingo, the capital of the southern state of Guerrero. But his beheaded body was found in a pickup truck Sunday; his head had been placed on the vehicle’s roof. Two rival drug gangs are fighting to control the city.