A report by experts from the Inter-American Commission on Human Rights, released Sunday, found that soldiers had been spotted at the crime scenes and that one intelligence agent had seen police clash with and detain a group of students.
Parents of the students and survivors of the tragedy have held protests in front of the headquarters of the army's 27th battalion in Iguala and have repeatedly questioned the military's role in the incident, on the night of September 26, when municipal police in the southern city of Iguala whisked away the students after shooting at their buses.
The report called on the authorities to investigate the actions of all security forces that night and whether any failed in their "obligation" to protect the students.
"It's clear that the government is responsible" because "lives could have been saved and they didn't," Santiago Canton, executive director of US-based RFK Partners for Human Rights, told AFP.
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While the commission's experts were never allowed to interview soldiers in person, they had access to the testimony of military personnel and reports of communications between regional security forces.
There, he reported that police cars intercepted one of the buses that the students, known as leftist radicals who hijack buses to head to protests around Guerrero state, had seized earlier in the night.
The agent, identified only be the initials "EM," said masked police officers tossed two tear gas grenades inside the bus to force the students to come out.
The police and students traded insults and officers warned: "Come out, or it will get worse for you."