When a mass grave containing 15 women, children and young men who lived 5000 years ago was found in a southern Polish village, a mystery surrounded their demise.
The discovery, made eight years ago, revealed that each person had been killed by blows to the head.
Conversely, the bodies were placed neatly together with gifts.
The researchers wondered why victims of a massacre would be buried with such respect.
But when they studied DNA from the remains recently, they encountered another surprise. All of the victims belonged to the same large family - three generations of them.
Researchers used radiocarbon dating, archaeological data, genomic and isotopic analyses to learn more about the bodies found in the village of Koszyce.
Their findings were published last week in the journal Proceedings of the National Academy of Sciences.
'By analysing ancient DNA from the skeletons, we were able to map each of the family relationships,' said Morten Allentoft, study author and evolutionary biologist at the University of Copenhagen.
'We can see that mothers are laid next to their children and brothers side-by-side. Those who buried the dead knew them well.'
Adult men are missing from the grave, with the exception of one man buried between his wife and son, but the researchers believe they know why.
'Our suggestion is that they weren't at the settlement when the massacre occurred and that they returned later, and subsequently buried their families in a respectful way,' Allentoft said.
The researchers believe there was a raid on the settlement between 2776 and 2880 BC.
The grave contained people from the Indo-European Globular Amphora culture, so named because of the shapes of the handled pots they made.
The Globular Amphora culture predated the Corded Ware culture, which comprised some neighbouring groups.
Corded Ware gets its name from the cord-like impressions on the pottery belonging to this culture.
The massacre may have been motivated by a fight for resources, the researchers said.
The two groups don't share ancestry, and the massacre occurred about the time the Corded Ware culture was rapidly spreading.
'We do not know who was responsible for this massacre,' said Hannes Schroeder, study author and archaeo-geneticist at the University of Copenhagen.
'But it is thought-provoking that it occurred 5000 years ago, as the late Neolithic Period was transitioning into the Bronze Age. During this period, European cultures were being heavily transformed by Yamnaya cultures migrating from the east. It is easy to imagine that these changes somehow precipitated violent territorial clashes.'
The grave includes eight males and seven females, dated and identified using dental and temporal bone DNA.
They represent four nuclear families. For the most part, they had brown eyes, dark or dark blonde hair and intermediate to dark skin, according to the study.
The skulls show cranial fractures but none on their arms and legs to suggest that they fought back in hand-to-hand combat.
The researchers believe they were captured and executed, a massacre typical of the violence associated with the Late Neolithic period, when populations were growing and resources were stretched.
Sometimes, whole communities were targeted. Other times, the men in a community were eliminated while women and children were taken as captives. In this case, the men either were already gone or fled during the raid.
One two-year-old is buried without his parents but placed next to his closest relations. A woman who wasn't genetically related to anyone in the grave was buried next to a young man, signifying their close relationship.
This speaks to the importance of family and social relationships in this Late Neolithic community, which seems to have been organised according to patrilineal lines of descent, according to the study.
Previously, researchers believed family structure was important in prehistoric societies. The careful placement of the bodies and the rich grave goods buried with them provide proof of that concept.
'We know from other grave site discoveries that violent conflicts played out among different cultural groups at this time,' said Niels N. Johannsen, study author and archaeologist at Aarhus University.
'However, they have never been as clearly documented as here. All the violence and tragedy aside, our study clearly demonstrates that family unity and care meant a lot for these people, some 5000 years ago, both in life and in death.'