Mexican federal forces disarmed a southern city's entire police force and took over security after officers were accused of colluding with a gang in violence that left 43 students missing. Monday's deployment in Iguala, 200km south of Mexico City, came after President Enrique Pena Nieto pledged justice in a case that has challenged his vow to tame drug violence. Authorities discovered a mass grave on a hill outside Iguala over the weekend containing 28 unidentified bodies, raising fears over the fate of the students, who were last seen in the city more than a week ago. Authorities say it will take at least two weeks to get the results of DNA tests to identify the badly burned corpses. The case could become one of Mexico's worst slaughters since the drug war intensified in 2006. Officials said more than 400 police and paramilitary police, along with an unspecified number of soldiers, had been dispatched to the scene. Carla Flora, a 40-year-old educator, welcomed the federal forces' arrival because "we were at the mercy of who knows who". "We felt unsafe. Organised crime was inside the municipal police," she said. Witnesses say several students, from a teacher's college known as a hotbed of protest, were whisked away in police vehicles on September 26 after officers shot at buses the youngsters had commandeered. Prosecutors say the Guerreros Unidos gang joined in the night of violence that left six dead, 25 wounded and 43 missing. Two hit men have confessed to killing 17 of the 43 students, saying they were told by Iguala's public security director to head to the scene of the shootings, while a gang leader told them to execute them, authorities said. State prosecutors said last week that they had detained 22 officers. Some 30 people in total have been detained, while the mayor and public security chief are on the run.