A 5,000-year-old grave found in Poland was discovered to be filled with extended family members and appears to have been buried by someone who knew them, according to new research.

The grave, filled with 15 individuals from the so-called Globular Amorpha culture, was first discovered by archaeologists in 2011, near the village of Koszyce in southern Poland.

An analysis of the ancient remains’ DNA found that all 15 victims -- who’d had their heads smashed in -- were related and that individuals were buried beside their close relatives, according to findings published on Monday in the journal PNAS.

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Researchers used the teeth and petrous bones of the remains to analyze their DNA.

They discovered there were four nuclear families among the buried, which included eight males and seven females. After studying their relationships, scientists then focused on how they had been buried.

“Closely related kin were buried next to each other: a mother was buried cradling her child, and siblings were placed side by side,” the scientists wrote. “Evidently, these individuals were buried by people who knew them well and who carefully placed them in the grave according to familial relationships.”

The researchers also found that older males and fathers were “mostly missing” from among the remains, “suggesting that it might have been them who buried their kin.” That also suggests the men were gone when their relatives were massacred, which would have made them vulnerable.

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According to researchers, the way they placed their dead relatives suggests how the people valued families in their culture -- something that had only been hypothesized about before.

“From the careful positioning of the bodies in the grave, it is clear that both nuclear and extended family relations were key to how people organized their lives, and that these relations represented major, normative values in Globular Amphora communities of this period,” scientists wrote.

“Although it has often been suggested that nuclear and/or extended family structures were important in many prehistoric societies, the archaeological and genomic data we have presented here provide actual proof that this was indeed the case,” researchers added.

Scientists also found that those buried were genetically different from neighboring people -- known as the Corded Ware groups -- who were expanding into the region around the time of the massacre.

“Although the reason for the massacre is unknown, it is possible that it was connected with the expansion of Corded Ware groups, which may have resulted in competition for resources and violent conflict,” the scientists wrote.