On the evening of 31 March 1922, six inhabitants of a small Bavarian farmstead, commonly referred to today as Hinterkaifeck ([ˈhɪntɐˌkaɪfɛk]), were murdered by an unknown assailant. Occurring approximately 70 kilometres (43 mi) north of Munich, the victims were Andreas Gruber (aged 63), his wife Cäzilia (aged 72), their widowed daughter Viktoria Gabriel (aged 35), Viktoria's children Cäzilia (aged 7) and Josef (aged 2) and their maid, Maria Baumgartner (aged 44). The murder weapon, a mattock, which was used on all six of the victims, was later recovered in the loft of the barn.

Immediately following the murders, the perpetrator lived with the six corpses for three days. During this time, they ate the food in the house, fed the animals on the property and used the fireplace. Four of the bodies were found stacked in the barn, the victims having likely been lured there one by one. Prior to the incident, other family members and a former maid had reported hearing strange noises coming from the attic, which led to said maid quitting the job. Considered one of the most gruesome and puzzling crimes in German history, the murders remain unsolved.

The farm at Hinterkaifeck, approximately 70 kilometres (43 mi) north of Munich in the German province of Bavaria, was built around 1863. The name Hinterkaifeck comes from its location: Hinter meaning "behind" in German, and Kaifeck being a small nearby hamlet of that name. Per its German name, the Hinterkaifeck farmstead was essentially located behind Kaifeck or, more precisely, about one mile north of it and bordered by dense woods.

Less than a year after the murders, the farm was demolished to remove any reminder of the violence that had taken place there. During the demolition, additional evidence was revealed, including a mattock covered in dried blood hidden in the attic and a penknife in the hay in the barn. After the demolition, a small concrete monument known as the Hinterkaifeck Andachtsstätte was erected close to where the farm once stood, serving as a memorial to the victims.

Andreas Gruber was born on 9 November 1858 in the Bavarian locality of Grainstetten. He married Cäzilia Sanhüter (b. 27 November 1849 in Gerolsbach) in 1886 and subsequently moved to Hinterkaifeck, ownership of which had been given to Cäzilia in 1877 as part of a divorce settlement with her first husband, Josef Asam. Cäzilia's earlier marriage had produced three children: Martin (b. 1879), Cäzilia (b. 1881) and Andreas (c. 1883). Her marriage to the elder Andreas produced two more: Viktoria (b. 1887) and Sophie (b. 1889).

Viktoria married Karl Gabriel, a native of Waidhofen, on March 11, 1914; ownership of Hinterkaifeck was transferred to the couple by her parents on that date. Several months later, following the start of the First World War, Karl was called up into the military. He was reported killed at the Battle of Arras on December 12, 1914. The union had produced a sole daughter, Cäzilia Gabriel, born on 9 January 1915.

In 1915, Andreas Gruber (the elder) and Viktoria (his daughter) were convicted in Neuberg district court of having engaged in an incestuous relationship between 1907 and 1910. Andreas was sentenced to one year in prison while Viktoria served one month. In 1919, Andreas was again reported to authorities after Viktoria admitted to her lover, a neighbour named Lorenz Schlittenbauer, that her newborn son, Josef Gruber, had been conceived through her father's continuing sexual abuse. Schlittenbauer withdrew his complaint against Andreas shortly after his arrest, only to reassert his claims while giving sworn testimony at Andreas's trial. The court did not consider Schlittenbauer's testimony to be a sufficient basis for conviction and allowed Andreas to be released. However, due to several incriminating statements made by Andreas during his own testimony, he did not receive compensation for his pre-trial detention. Schlittenbauer agreed to adopt Josef as his own son.

In the months prior to the killings, strange occurrences were reported in and around Hinterkaifeck. Kreszenz Rieger, the Gruber family's original maid, had quit about six months before the murders; it has been widely claimed that Rieger's reason for leaving was that she had heard strange noises in the attic, believing the house to be haunted. In March 1922, Andreas found a strange newspaper from Munich on the property that he could not remember purchasing, initially believing that the postman had lost the paper. This was not the case, however, as no one in the vicinity subscribed to the newspaper.