An independent probe into the disappearance of 43 Mexican students put a spotlight on the army's failure to protect them from crooked police despite knowing about the attacks.

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.

While the police forces of Iguala and neighboring Cocula were the "direct assailants," agents from the army and federal police "were present at different times" when the students were detained and disappeared, the independent report says.

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.

- Agent 'EM' on motorcycle -

"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.

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.

A military intelligence agent wearing civilian clothes rode on a motorcycle to the scene of one confrontation in front of Iguala's courthouse, according to his statement to investigators.

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."

The agent reported seeing 10 students taken out of the bus, handcuffed and thrown "aggressively" to the ground.

- Would have backed police -

Mexican prosecutors have only accused Iguala's police of wrongdoing, saying officers abducted the students and handed them over to a drug gang that killed them and incinerated their bodies at a landfill. But the independent experts said there was no scientific proof they were incinerated there.

The independent investigators had access to reports from the so-called C-4 command center that filters communications between federal, state and local security forces in the region.

The reports showed that the army and federal police had been monitoring the students from the moment they left the state capital, Chilpancingo, and headed toward Iguala on hijacked buses.

After the shootings, soldiers went to a hospital where wounded students were taken as well as the police station to check if they were there.

Others secured a location where two students were shot dead.

In July, Defense Minister Salvador Cienfuegos, a general, said that soldiers had been busy responding to a different emergency, involving a tanker accident, during the attacks against the students and only went to the hospital when they arrived at the scene.

He said that if the soldiers had come during the confrontations, they would have likely backed the police.

"Good thing we didn't have (the personnel available) ... because if they had come out we would have created a bigger problem," Cienfuegos told Excelsior newspaper.

Raul Benitez Manaut, a security expert at the National Autonomous University of Mexico, said that if military intelligence agents were present during the attacks, "it's a good enough reason to completely reopen the investigation."