Mexican soldiers who participated in a June 2014 mass slaying of 12 civilians, believed to be cartel members, received standing orders to kill the suspected criminals, the Associated Press said based on a report by a local human-rights group.

Military documents showed that high-ranking officers had approved of the alleged executions, and lower-ranked army members involved in the incident could well argue that they were merely complying, the Miguel Agustín Pro human rights center warned.

"Soldiers should operate on a mass scale at night and reduce daytime activities, with the aim of killing criminals at night," read standing orders signed on June 11, 2014, by Lt. Col. Sandro Díaz Rodríguez on behalf of the command of the 102 Infantry Battalion, the AP noted.

The report draws new scrutiny to the Mexican government's controversial deployment of the armed forces to combat drug cartels and urges investigators to look into the potential responsibility of high-ranking officers in the 2014 crackdown in Tlatlaya, a  small town in the southeast of the state of Mexico, the Australian Broadcasting Corporation detailed.

The human-rights group put the total death count in the incident at 22 and reiterated its claim that the suspected cartel members were murdered in a warehouse. The army, on the other hand, reported on June 30, 2014, that it killed the suspects in a shootout, even though only one soldier was injured.

Three soldiers have since been charged with the murder of eight people in the incident, while four others, including an officer, have been accused of violating their public-service duties. But Mario Patrón Sánchez, who leads Miguel Agustín Pro, insisted that the judicial process was insufficient.

"We are facing a completely illegitimate and illegal order that deserves an investigation over the responsibility of the chain of command. Who issued the order? Who authorized the order?" he asked.

A similar incident during which 42 suspected criminals were killed in Michoacán state earlier this year, also raised concerns about potential extra-judicial killings, the Associated Press noted.