9. The Hinterkaifeck murders
This one is straight out of your nightmares. Most likely, the killer(s) were living in the house at the same time that the family was and they never knew it.
From Wikipedia:
Hinterkaifeck was a small farmstead situated between the Bavarian towns of Ingolstadt and Schrobenhausen, approximately 70 kilometres (43 mi) north of Munich. On the evening of March 31, 1922, the six inhabitants of the farm were killed with a mattock. The murders remain unsolved.
The six victims were the farmer Andreas Gruber (63) and his wife Cäzilia (72); their widowed daughter Viktoria Gabriel (35); Viktoria’s children, Cäzilia (7) and Josef (2); and the maid, Maria Baumgartner (44).
A few days prior to the crime, farmer Andreas Gruber told neighbors about discovering footprints in the snow leading from the edge of the forest to the farm, but none leading back. He also spoke about hearing footsteps in the attic and finding an unfamiliar newspaper on the farm. Furthermore, the house keys went missing several days before the murders. None of this was reported to the police prior to the attack.
Six months earlier, the previous maid had left the farm, claiming that it was haunted; the new maid, Maria Baumgartner, arrived on the farm on the day of the attack and was killed hours later.
Exactly what happened on that Friday evening cannot be said for certain. It is believed that the older couple, as well as their daughter Viktoria, and her daughter, Cäzilia, were all lured into the barn one by one, where they were killed. The perpetrator(s) then went into the house where they killed two‑year‑old Josef, who was sleeping in his cot in his mother’s bedroom, as well as the maid, Maria Baumgartner, in her bedchamber.
On the following Tuesday, April 4, neighbors came to the farmstead because none of its inhabitants had been seen for a few days. The postman had noticed that the post from the previous Saturday was still where he had left it. Furthermore, young Cäzilia had neither turned up for school on Monday, nor had she been there on Saturday.
