Muslim cattle herders are suspected of killing four police officers in the northeastern Nigerian state of Adamawa, a police official said on Friday.

Reuters reports that the four officers were killed on Thursday night defending a village in the Numan region from the herdsmen, who attacked the settlement as a reprisal for an earlier deadly clash, said Othman Abubakar, a police spokesman for the state.

In the earlier clash, unidentified attackers killed more than 30 cattle herders in Numan.

Numan has recently become a flashpoint for clashes between the herders and Christian farmers, which occur frequently across broad swathes of Nigeria, as each group contests the other’s rights to land for pasture and agriculture.