Friedrich Heinrich Karl "Fritz" Haarmann was born on 25 October 1879 in Hanover, Germany. By the time police arrested him on 23 June 1924, he had become one of Weimar Germany's most notorious serial killers, systematically preying on vulnerable young men and boys for over six years.

Haarmann typically selected his victims from among the vagrants and runaways who congregated around Hanover's train station. These were young men with few connections, few people looking for them—ideal targets for a predator operating in plain sight. Between 1918 and 1924, with the most active period spanning 1919 to 1924, he lured them to his apartment where he sexually assaulted them. His method of killing was particularly brutal: manual strangulation combined with biting through the victim's Adam's apple or throat. After murdering his victims, Haarmann dismembered their bodies and disposed of the remains in the River Leine.

The sheer scale of his crimes only became apparent during the investigation. Police dredged over 1,500 human bones from the river, their flesh already removed by the water. Detectives also recovered victims' clothing and possessions from Haarmann's apartment—evidence that would prove crucial to the prosecution.

Haarmann did not act entirely alone. Hans Grans, his accomplice, helped select victims and profited by selling the dead men's belongings. This partnership extended Haarmann's killing spree and added another dimension to his crimes: commercial exploitation of his victims even in death.