Federal agents have taken control of a violence-wrecked city in Guerro, Mexico, after a mass grave was uncovered and local police were linked to the slaughter.

Federal troops took charge on Monday, tasked with keeping order in Iguala and helping to look for 43 students missing following violence on September 26, in which six died.

State officials were still working yesterday to see if any of the missing were among 28 bodies found in pits  on a hillside outside the city on Saturday night, with some so badly damaged it could take a fortnight before work is complete.

President Enrique Pena Nieto said he dispatched federal security forces to “find out what happened and apply the full extent of the law to those responsible.”

Guerrero State Prosecutor Inaky Blanco said so far there was no known motive for the attack, but officials have alleged that local police were in league with a gang called the Guerreros Unidos.

The gang has reportedly put up a banner demanding the release of 22 police officers arrested in connection with the disappearance, threatening that “the war has started” if their demands aren’t met.

The violence broke out when police opened fire on a bus hijacked by protesting students from a nearby teaching college on September 26, killing three.

Masked gunmen later opened fire on buses and taxis carrying a football team, killing three more.

Video footage subsequently appeared showing police rounding up and arresting students, who haven’t been seen since.