The deliberate suffocating of over 60 men and boys stuffed into a baking hot shipping container in South Sudan is a war crime, Amnesty International said Friday.
In a report detailing the atrocity by government soldiers for the first time, the London-based rights watchdog called for the perpetrators to be prosecuted.
The killings, which the government has denied, took place in a Catholic church compound in the central town of Leer in October 2015, Amnesty said.
It was first reported last month by the Joint Monitoring and Evaluation Commission (JMEC), a regional ceasefire body pushing peace efforts.
Amnesty's report, based on 23 eyewitnesses who saw the men and boys forced into the container with their hands tied or saw the bodies later dragged away and dumped, provides more details and attributes blame for the killings.
"Witnesses described hearing the detainees crying and screaming in distress and banging on the walls of the shipping container, which they said had no windows or other form of ventilation," the report said.
Temperatures in the northern battleground state of Unity regularly top 40 degrees Celsius (100 degrees Fahrenheit).
"They said that civilian and military officials had direct knowledge that the detainees were in distress and dying but did nothing to help them," the report said.
Relatives of those killed said the victims, "were cattle keepers, traders and students, not fighters," according to Amnesty.
Amnesty said one witness saw troops open the container, remove four corpses and then, "close the container again on the remaining detainees who were still alive inside."
The earlier JMEC report said that those found alive were then killed, and that the only survivor was an eight-year old boy.
"We could see the people inside and they were not alive," one witness told Amnesty.
"What we saw was tragic... the container was full of people. They had fallen over one another and on to the floor. There were so many people."
The bodies were later dumped in an open field, with Amnesty researchers finding, "remains of many broken skeletons still strewn across the ground."
After winning independence from Sudan in 2011, South Sudan erupted into civil war in December 2013, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.
Both the government and rebel sides have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.