In 2013, scientists broke the news that they had found the remains of nearly 30 early human skeletons buried deep at the bottom of a 43-foot (13-meter) shaft inside a Spanish cave dubbed Sima de los Huesos. Questions quickly arose as to how the bodies ended up in such an inaccessible location, as there was no evidence the cave served as a dwelling place. Now, a group of scientists is arguing that the owners of the remains may have been placed there en masse—after being killed by fellow early humans.

After piecing together a complete skull from 52 bone fragments found in the cave, the authors of the new study (published this week in PLOS One) concluded that the skull’s owner was a young adult who lived some 430,000 years ago, during the Middle Pleistocene epoch. They were unable to determine whether the individual was male or female, but what they did find was stunning: Two cranial fractures, located on the left side of the forehead above the eye, each nearly an inch wide and each showing a similar notch in its outline.

The scientists found no indication that either of the fractures had begun to heal by the time of the individual’s death, or that predators had a chance to eat any of the remains. They also don’t believe that environmental damage could have caused the symmetrical wounds. As paleontologist Nohemi Sala, who led the analysis at the Instituto de Salud Carlos III in Madrid, told Reuters: “Based on the similarities in shape and size of both the wounds, we believe they are the result of repeated blows with the same object and inflicted by another individual, perhaps in a face-to-face encounter.”

This was not a case of accidental contact, the scientists argue, but was “more likely to be the result of interpersonal violence.” According to their findings, the skull’s owner was likely bludgeoned to death with some hard object—perhaps a wooden spear or stone hand-ax, or even a plain old rock. The attacker was probably right-handed, given the location of the wounds, but his or her identity—and motive for murder—will almost certainly never be known.

The evidence pointing to homicide for the owner of Cranium 17 (as the skull was identified) might explain, moreover, why close to 30 bodies were found together at the bottom of one Spanish cave. As Sala and her colleagues write in their report: “The only possible manner by which a deceased individual could have arrived at the Sima de los Huesos site is if its cadaver were dropped down the shaft.” If their suspicions are correct, the scientists may have found evidence of the world’s first mass murder—some 230,000 years before the first Homo sapiens, or early humans, appeared in Africa.

That violent behavior—including murder—predates modern human society may not come as a shock to anyone. For their part, the researchers took it in stride. “Violence is a very usual behavior for animals,” Sala told the Guardian. “It’s not surprising that interpersonal violence took place [among early humans].”