Archaeologists working in Syria have unearthed the remains of dozens of youths thought to have been killed in a fierce confrontation 6,000 years ago.
Only a fraction of the burial pit has been excavated
According to Science magazine, the celebrating victors may even have feasted on beef in the aftermath.
The findings come from northeastern Syria, near Tell Brak, one of the world's oldest known cities.
More than 30 years of continuous excavation have revealed the site's remarkable sophistication.
Studies by British and American archaeologists published in the journals Antiquity and Science suggest Tell Brak was a flourishing urban centre at the same time as better known early cities from southern Iraq.
The work also indicates that, unusually, the Syrian city grew from the outside-in, rather than the inside-out.
A third paper, due to be published in an upcoming edition of the journal Iraq, details the burials at Tell Majnuna, 0.5km from the main urban site at Tell Brak.
The surface of Tell Brak is covered with broken pottery and other debris
Two mass burial pits have been excavated at this site. The first has so far revealed the bones of 34 young to middle-aged adults. Thus far, only a small portion have been excavated.
"There could be hundreds and potentially thousands," said Augusta McMahon, an archaeologist at the University of Cambridge, UK.
At least two skulls show signs of injuries that could have caused death. The absence of feet and hand bones and the fact that many of the skulls apparently rolled off when they were tossed in the pit hints that they were left to decompose before burial.
A mass of pottery, mostly vessels for serving and eating, along with cow bones were also found lying on top of the skeletons.
The experts interpret this as evidence for a large feast, according to the news report in Science.
A second mass burial pit has been found about 12m away. At least 28 individuals have been uncovered from this location.
Dr McMahon said she did not know whether the victors were defending or attacking Tell Brak.
"We need at least another season to understand what happened," said Joan Oates, an archaeologist at Cambridge and project director at Tell Brak.
She estimates that the Majnuna incident took place in about 3,800BC.
Tell Brak is a 40m-high, 1km-long archaeological mound in what would have been northern Mesopotamia.
Jason Ur, an anthropologist at Harvard University in Cambridge, US, and colleagues have carried out a study of the site's evolution by determining the placement and age of artefacts uncovered there.
Instead of growing from a populated urban centre in an outward direction, Tell Brak began as small settlements with space between them.
Eventually, the population grew more dense and moved towards the centre.
This, researchers say, supports the idea of a lack of centralised authority at Tell Brak.
It also suggests that more than one model of city development should be considered in studying Mesopotamian archaeology.