This paper examines the notable double murder case in Sweden, which remained unsolved for 16 years until it was significantly advanced by the application of genetic genealogy. The Linköping double murder case resulted in the deaths of an eight-year-old child and a 56-year-old lady, both of whom were fatally stabbed on a public street in Linköping, Sweden. Despite the culprit leaving DNA at the crime site, the 15-year investigation, which involved over 9,000 interrogations and bulk DNA screens of over 6,000 males, yielded no actionable information about the assailant's identity. The Swedish Police Authority became interested in investigative genetic genealogy (IGG) when it was successfully implemented in the United States. Through extensive DNA analysis—encompassing whole-genome sequencing and genotype imputation DNA datasets were developed and explored within genealogy platforms such as Family Tree DNA and GEDmatch. This diligent research identified several distant relatives, facilitating the construction of family trees. As a consequence of this genealogical inquiry, two brothers emerged, one of whom matched the samples collected from the crime site using conventional STR profiling. He later acknowledged to the killings during an initial police interrogation and was found guilty. The study emphasises the importance of thorough forensic analysis and investigative approaches in successfully resolving this case. It highlights the importance of collecting and analyzing DNA evidence, gathering witness testimonies, and leveraging genealogical databases.