In this August 2012 photo provided by Marta Mirazon Lahr, researcher Frances Rivera, right, Michael Emsugut, left, and Tot Ekulukum excavate a human skeleton at the site of Nataruk, West Turkana, Kenya. This skeleton was that of a woman, found lying on her back, with lesions on her neck vertebrae consistent with a projectile wound. She also had multiple fractures on one of her hands. Writing in a paper released Wednesday, Jan. 20, 2016, by the journal Nature, scientists said it’s one of the clearest cases of violence between groups among prehistoric hunter-gatherers. (Marta Mirazon Lahr via AP)The causes of death of the men with arrow wounds in their backs, or pieces of obsidian blades in their skulls, were easy enough. But, the pregnant woman with her hands and feet bound still remains a mystery.

Scientists recently discovered some of the most well-preserved signs of a violent massacre that have ever been found in skeletal remains of ancient man some 10,000 years ago. Their findings could represent the oldest forensic evidence ever studied, and may shed light on the earliest analyzed signs of human warfare. 

The remains were found at the edge of an ancient lagoon in Nataruk, in modern-day Kenya.  Researchers found 12 relatively complete skeletons back in 2012—27 sets of remains were found in all. Ten of the 12 showed signs of violent wounds that would have likely caused immediate death to the victims.

Watch this video of the excavation:

One of the four adult females in the group was in the late stages of pregnancy, or had recently given birth, according to the study. Her remains showed no signs of violent trauma, although the position of her body suggests that she was likely bound at the time of her death.

The findings, “offer a rare glimpse into the life and death of past foraging people, and evidence that warfare was part of the repertoire of inter-group relations among prehistoric hunter-gatherers,” according to the authors.

Researchers said the remains showed no signs of a ritual or purposeful burial, and were likely preserved naturally by the “particular conditions” of the lagoon at the time.

The reason for the violence cannot be determined from the wounds, although the researchers speculated that a territorial conflict over food, or other necessary resources, was the most likely cause. Or, they said, the two groups might have just not liked each other.

The findings were published in the journal Nature last month.