MEXICO CITY -- An army officer and seven soldiers face disciplinary action for their participation in the killing of 22 people in rural southern Mexico, but the army remained mum Friday on what roles they played.

The Mexican Defence Department said he eight were involved in the June 30 incident in San Pedro Limon, an encounter that the military initially reported as a shootout but that a witness has described as a massacre.

The National Human Rights Commission has requested a report on the arrests, investigator Marat Paredes said Friday. The army's statement did not reveal the names of those detained nor their battalion. They were being held at a prison in Mexico City on charges of crimes against military discipline, disobedience and dereliction of duty."

The statement issued Thursday night said the military charges are being pursued "independently of the investigations that civil authorities are carrying out under their jurisdiction," leaving open the possibility of charges in civilian courts.

President Enrique Pena Nieto said this week that the federal Attorney General's Office is also probing the incident.

Shortly after the confrontation, the army reported that 22 suspected criminals had been killed in a firefight with troops. Only one soldier was wounded. The official version came into question when The Associated Press visited the scene days later and found no sign of a prolonged battle.

At least five spots inside the warehouse where the bloodshed occurred showed the same pattern: One or two closely placed bullet pocks, surrounded by a mass of spattered blood, giving the appearance that some of those killed had been standing against a wall and shot at about chest level.

Last week, a woman who says she witnessed the events told the AP that only one person died in the initial confrontation and the rest were shot after surrendering. The witness said the dead included her 15-year-old daughter, Erika Gomez Gonzalez, who had been wounded in the leg and was lying on the ground when she was killed.

The national newspaper La Jornada published photos Friday showing bloody bodies, purportedly in the immediate aftermath of the shooting, with investigators and military personnel still at the scene.

The bloodstains on the cinderblock wall, evidence markers and debris shown in the pictures match those that AP journalists photographed days after the deaths were reported by the army.

Among the dead is a girl lying on her back in the manner that the witness described. Her brother identified her as Erika, including identifying her bloody and mud-covered clothing.

"I bought her that T-shirt," said Saddam Guzman Gomez. "It's not true that she was shooting. They planted that gun. It looks like she was crawling."

Many of the dead are shown sprawled beside the wall of the warehouse. All pictured are holding or lying near assault rifles, which in some cases appear propped against the bodies. Most have mud on their knees, indicating they may have been kneeling or lying face down.

The AP is not distributing the photographs because it cannot determine their source.

A plain yellow envelope containing the photos on a USB memory stick was sent anonymously on Wednesday to MVT, a local news agency in Mexico state, said the agency's director, Mario Vazquez. He checked the photos with those his agency took the day of the shooting and concluded it was the same scene.

On Friday, Mexico's secretary of the interior defended the armed forces in a meeting in the lower house of Congress.

"If there was something they have done wrong, it would be the exception," Miguel Angel Osorio Chong said of the eight detained. "It's an isolated incident and doesn't reflect the behaviour of our great army and navy in Mexico."

Until recently, officials have stood by the initial account of a fierce firefight.

In July, the state of Mexico prosecutors' office released a statement saying there was "no evidence at all of possible executions." The office said it found ballistic evidence of "crossfire with a proportionate interchange of gunshots."

The state government refused to release autopsy reports that the AP requested under Mexico's freedom of information law, declaring them state secrets to be guarded for nine years.

The federal Attorney General's Office last week said that so far it had not found evidence corroborating the witness' account.

But this week, President Enrique Pena Nieto said in New York that the Attorney General's Office was diving into the investigation and would answer all questions. Gen. Salvador Cienfuegos, the secretary of defence, said in a public speech on Thursday -- the same day of the eight detentions -- that soldiers must respect human rights despite whatever risks they may face in the line of duty.

"Any behaviour that departs from that rule will be taken to the proper authorities," he said.

The U.S. State Department and international rights groups have called for thorough investigations. Human Rights Watch said the incident could prove to be one of the "most serious massacres in Mexico."

"One by one, they're dismantling the official version that sought ... to cover and protect once again those responsible for crimes committed on the part of military forces," said Jose Miguel Vivanco, the organization's director for the Americas.