Soldiers murdered at least 12 and probably 15 people at a warehouse in southern Mexico last summer, the government’s human rights commission said in a sweeping indictment of attempts by the military and civilian prosecutors to cover up the crimes.

The commission’s report exposed the constantly changing government versions of the killings and the brutality with which they were carried out, and could spell the end of prosecutors’ efforts to first deny the killings, and then blame them on three rogue soldiers.

The commission president, Raúl Plascencia, called it “one of the most serious human rights violations that can be committed” and issued a formal recommendation demanding prosecutors investigate attempts to cover up the extrajudicial killings of the suspected gang members. Three of the victims were adolescents.

Plascencia said bodies were moved and the crime-scene was altered to mimic a shootout, which was the army’s initial explanation of what happened. “The scene was so altered that some bodies were moved, and weapons were placed on all the dead bodies lying in the dirt,” he said on Tuesday.

The human rights investigation turned up gruesome details, including that someone had twisted the head of one suspect until his neck broke, Plascencia said. Four other bodies had marks of having been beaten with blunt objects before they were killed.

Bullet marks and other evidence indicated that seven others among the dead were lined up near three of the warehouse’s walls and shot “when they were disarmed and were not resisting,” Plascencia said.

None of the three government agencies blamed in the report – the federal attorney general’s office, the defence department and the State of Mexico state prosecutors office – would give any immediate reaction to the report.

The demand by Plascencia’s commission for an investigation into the coverup is not binding, but the government must publicly explain its reasons if it does not comply.

The report constituted the third official version of what happened at the abandoned warehouse in the town of San Pedro Limón in southern Mexico.

The army initially said all 22 suspected gang members died during a gunbattle with soldiers, but the attorney general, Jesús Murillo Karam, said recently that eight of the 22 had presumably been killed after surrendering. Murrillo Karam said only three soldiers had actively participated in the killings, but Plascencia said that at least five soldiers were involved and that the participation of a sixth soldier – possibly a higher-ranking officer – should be investigated.

Perhaps some of the most chilling evidence was found against civilian prosecutors in the State of Mexico, where the killings occurred.

The report said prosecutors either tortured the three women who survived the killings to force them to concur with the army’s version of events, or allowed unidentified people to torture the women with beatings, partial asphyxiation, sexual aggression and threats of rape if they dared speak out about what really happened.

“They pulled her hair, punched her in the ribs, and with a shopping bag they covered her nose and mouth, asphyxiating her,” the report cites one of the survivors as saying. “It is clear that we are facing a case of aggravated torture.”

The three women were at the warehouse, but were spared by soldiers because they told the troops they were kidnap victims. However, two of them, apparently gang associates, were later charged with weapons possession and are now in a federal jail. The third woman, who was trying to rescue her daughter from the warehouse, was released and later spoke to reporters about the horrifying scene.

The commission said state prosecutors, who said in July that they had found no evidence of extrajudicial killings, drew up dubious autopsy reports and did not take adequate crime scene photographs. It said federal prosecutors did not even start investigating the case for almost three months, despite its duty to do so immediately.

The army’s version of a shootout almost immediately came under question because only one soldier was wounded.

The Associated Press visited the scene three days after the shootings and found evidence suggesting some of the dead may have been murdered. The woman who had witnessed the killings later told the AP that only one man died in an initial shootout and all the others were shot after surrendering. The witness’ account of the mistreatment and threats of rape at the state prosecutor’s office matched the description in the commission’s report.

Three soldiers face civilian murder charges and eight soldiers face military discipline charges in the case.

Federal prosecutors say the three entered the warehouse and killed suspects who were no longer fighting and that the other five soldiers, including an officer, stayed outside.

But Plascencia’s report depicted a scene of such brutality – and such extensive efforts to cover it up – that it raised questions on whether three soldiers alone could have carried out the acts.