Sometime in the late 5th century, a brutal mass killing took place at the settlement known as Sandby Borg on the Swedish island of Öland. It appears that the residents had just enough warning of the attack to hide their valuables. After the slaughter, the bodies left where they lay, the houses closed up, and the area abandoned and apparently taboo for years.

That makes it a valuable portrait of daily life in Scandinavia. “It’s compelling because people were killed inside the houses, and then the killers went out, locked the doors, and left,” said Swedish archaeologist Ulf Näsman. The attackers left even the livestock behind.

The reason for the attack is still a mystery. The usual signs of a battle or siege are missing — in fact the remains of the victims are oddly bare. “There is no dress, such as belt buckles [on the skeletons],” Näsman says. “Were they caught unawares at night? Maybe they were nude or in night dress and taken by surprise.”

And, strangely, two of the victims were found with sheep or goat teeth in their mouths. “It wasn’t enough to kill them and leave them in their houses,” says archaeologist Helena Victor, who directs the excavation. “It’s really, really ugly treatment.”