MEXICO CITY — The mystery of the whereabouts of 43 college students reported missing after an outbreak of violence in southern Mexico deepened Tuesday after the authorities said initial testing had shown that none of the students were among the 28 bodies found in mass graves.

At the same time, the authorities said they were checking newly found graves to see if the undetermined number of remains found were those of the students.

The students were reported missing on Sept. 26 after gunfire by the police and masked attackers left six dead and several wounded in the city of Iguala in Guerrero State. Three students, part of a large group collecting donations for school and then stealing buses to travel to an Oct. 2 demonstration protesting cuts to their state-financed teachers college, were killed.

The federal attorney general, Jesús Murillo Karam, revealed the findings on the first mass graves as he reported additional arrests in the case, including the seizure of 14 police officers in Cocula, a small town to the southwest of Iguala. The officers, officials said, had confessed to delivering students detained in Iguala to an organized-crime gang, Guerreros Unidos, which has infiltrated several local police forces. Already, 22 officers in Iguala have been detained on suspicion of having also turned over students to the gang.