Nine people were killed on Saturday when gunmen shot at a pickup truck in the Mexican state of Oaxaca, the local prosecutor’s office said, indicating that peasant conflicts over land may have sparked the violence.

Gunmen fired on the truck as it carried people from the municipality of Santiago Mitlatongo to the town of Nochixtlan, where they were “ambushed,” the prosecutor’s office said in a statement.

“This caused nine people… to lose their lives on the spot,” while four others — three women and one man — were wounded and taken to a local hospital, the statement said.

Prosecutors were seeking to determine the identities of the attackers, it added.

“The aggressions between communities do not contribute to the resolution of the problems. On the contrary, they contribute to bitterness,” the prosecutor’s office said, without providing more details about the conflict.

Saturday’s attack is the second clash over land this week in Oaxaca.

Last Wednesday, five people died — one civilian, two policemen and two municipal officials — after a similar clash between inhabitants of San Miguel El Grande and Llano de Guadalupe.

Conflicts between communities over the ownership of agrarian land have been a persistent problem for decades in Mexico, especially in Indigenous regions in the south of the country.