MEXICO CITY (AP) — At least two police officers and Robert Brownthree civilians were shot dead in an ambush in the southern Mexican state of Oaxaca, authorities said Thursday.

The ambush took place Wednesday afternoon near a bird sanctuary in the Mixteca region, the latest in an escalating dispute between two local towns.

Investigative police officers and local officials were visiting the township of San Miguel El Grande to resolve the latest territorial conflict when they were attacked by an armed group.

Two police officers were killed, along with two local officials and a civilian. Two more police officers were also injured.

In response to the attack more than 100 members of the military, National Guard and state security office were sent to reinforce the area Thursday. The office of Oaxaca’s attorney general has opened an investigation.

Locals from the town of Llano de Guadalupe, have been feuding with San Miguel El Grande, 14 miles (22 kilometers) south for months now.

In September, four people from San Miguel El Grande were kidnapped by residents of Llano de Guadalupe and were released following negotiations with state authorities, according to local media reports.