Operating in the rural Twente region alongside his wife and son, Annink built a years-long pattern of robbery and suspected murder that went largely unchecked until an outsider — a Hanoverian merchant pursuing a missing relative — finally brought enough evidence to force an arrest. The family's crimes were localized but sustained, and the case left an unusual material trace: the restraining chair constructed specifically to hold him during his 114-day detention survives in a museum today, a reminder of how seriously authorities ultimately took the threat he posed.

His method was methodical rather than frenzied — killing for financial gain, then using sulphuric acid to dissolve the evidence before forging signatures and liquidating his victims' assets. The combination of murder, fraud, and near-total destruction of physical evidence made him one of the more forensically calculated killers of postwar Britain, and the case became a landmark in the history of forensic investigation.