IGUALA, Mexico — Mexico sent federal agents to take over security in a troubled city in southern Guerrero state after the discovery of a mass grave and charges that local police conspired with a criminal gang to kill and disappear students.

The newly created preventative unit of the federal police was tasked Monday with keeping order in Iguala and helping search for the 43 students still missing following the Sept. 26 attack, in which six people died.

As state officials worked to determine whether any of the missing were among 28 bodies found over the weekend in a clandestine hillside grave, President Enrique Pena Nieto called the deaths “outrageous, painful and unacceptable.”

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.

Investigators said video showed officers taking away an undetermined number of students, who had gone from a rural teachers college in Ayotzinapa to the city to solicit donations.

Speculation abounded among parents and local residents as a banner appeared in the name of the Guerreros Unidos. It demanded that 22 police officers detained in connection with the attack be released within 24 hours, and warned of consequences otherwise: “The war has started.”

The federal takeover came amid rising international concern over the Iguala incident and another possible case of a mass killing involving Mexican authorities.