The execution

Around 2,800 BCE, in what is now southern Poland, a family group of fifteen individuals associated with the Globular Amphora culture (GAC) were massacred. They were probably captured and executed, because each victim was killed with a blow to the head from the same type of weapon, possibly a stone axe, and lacked defensive wounds. The dead were mostly women and children. They were buried in a mass grave, but with great care and very likely by someone who knew them well.

This Late Neolithic mass grave is the focus of a new ancient DNA and archeological research paper at PNAS by Schroeder et al. (see here). The authors tentatively attribute the massacre to the Corded Ware culture (CWC) people, who were expanding rapidly at the time across much of Europe from their homeland on the Pontic-Caspian steppe.

The CWC people may or may not have been responsible; we'll never know for sure. The perpetrators could just as easily have been a competing GAC family group.

In any case, it's interesting to see that the GAC males belong to Y-chromosome haplogroup I2a-L801. This is today a rather uncommon subclade of I2, and almost exclusively found in Germanic-speaking populations, especially Scandinavians. To me this suggests that some Polish GAC males were incorporated into Indo-European-speaking CWC populations that ended up in Scandinavia, and their paternal lineages eventually became a part of the Proto-Germanic gene pool. Admittedly, though, that's just one of many possible scenarios.

See also...

Late PIE ground zero now obvious; location of PIE homeland still uncertain, but...

Corded Ware people =/= Proto-Uralics (Tambets et al. 2018)

Inferring the linguistic affinity of long dead and non-literate peoples: a multidisciplinary approach