The authorities in Guerrero, one of Mexico’s most violent states, are searching for 57 students from a teachers college who have been missing since Friday when gunmen fired on a demonstration they were attending, killing three students and three others nearby. The gunfire also wounded 25 people. The Guerrero state prosecutor, Iñaky Blanco Cabrera, said that witnesses identified the gunmen as local police officers in Iguala, where the protest occurred, and that more than 20 had been detained. Mr. Blanco described the episode as an instance of “undoubtedly excessive force” by the officers. The federal interior secretary, Miguel Ángel Osorio Chong, called the conduct of the police “incredible” and said federal authorities would join the investigation. Witnesses said police officers had put the missing students on a bus and escorted them from the scene.