Mexico ? Six city police officers were arrested Friday in the killing of a mayor in northern Mexico, as the country’s escalating drug violence targets more public officials.

The suspects included the officer who guarded the house where Santiago Mayor Edelmiro Cavazos was seized on Sunday. The officer had said he was kidnapped with the mayor and later freed unharmed.

Adrian de la Garza, head of the police investigations agency in Nuevo Leon state, told a news conference that the police officers received 6,000 pesos ($700) per month to cooperate with criminals “in different ways and different affairs,” with some allegedly acting as lookouts.