Mexican authorities on Tuesday said they found 19 bodies buried in a mass grave in the southwestern state of Guerrero, where last year 43 students were kidnapped and apparently massacred.

The unidentified bodies, some of which had been burnt, were buried at the bottom of a 1,640-foot deep ravine in Chichihualco, a town northwest of state capital Chilpancingo, a spokesman for the local prosecutor’s office said.

The bodies were first discovered last week, he added.

Guerrero is one of the most violent states in Mexico, where more than a dozen drug gangs are fighting to control poppy production and other criminal enterprises.