As far back as the 11th century, a disturbing myth took hold across Eastern Europe: some people who died would claw their way out of the grave as blood-sucking monsters that terrorized the living. Evidence of anti-vampire rituals—a metal rod hammered through a centuries-old skeleton, for instance—is widespread in the region. But an important question remains: How did the living decide who was at risk of becoming a vampire?

One popular hypothesis among scholars is that strangers, new to town, might have been targeted as vampires. But determining which long-buried corpses belonged to locals and which to foreigners is no easy task.

To put this hypothesis to the test, researchers turned to biogeochemical analysis of bones found in vampire graves from a cemetery in northwestern Poland. The researchers discovered six unusual skeletons, all buried between the 17th and 18th century. They all had either sickles placed across their neck or body or large rocks stacked under their chin—signs that whoever buried these bodies took extra precautions to keep them in the ground. All of the hundreds of other bodies in the cemetery, on the other hand, were perfectly normal.

The researchers analysed radiogenic strontium isotope ratios—an indicator of diet—extracted from the tooth enamel of 60 skeletons (including the six vampires) to see what these people had eaten during their lives. The results surprised them: all six of the vampires were likely from the area.

"These individuals were not suspected of becoming vampires due to their identity as non-locals, but instead, were distrusted within some other, additional societal context as members of the local community," the researchers report today in the journal PLOS ONE. In other words, these weren't creepy strangers who had wandered into town; these were loved ones and neighbors.

What could have caused a parent to suspect a daughter of becoming a vampire or a wife to worry that her husband's body might return from the grave? The authors do have one guess: it could be that those labeled vampires were the first victims of cholera epidemics—a common problem at the time those six persons died. As the study's lead author, Lesley Gregoricka, hypothesized: "People of the post-medieval period did not understand how disease was spread, and rather than a scientific explanation for these epidemics, cholera and the deaths that resulted from it were explained by the supernatural – in this case, vampires."