Federal agents in Mexico are taking over security in a city where a mass grave was discovered. Local police are accused of working with a criminal gang to kill students or make them disappear.

The unit of federal police was tasked with keeping peace in Iguala and helping search for the 43 students still missing following an attack back in September. Six students were confirmed dead in the rash of violence.

Authorities are still trying to figure out if the 28 bodies found in the mass grave were students involved in last month’s attack.

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