How advanced genetic analysis led to Johnny Vassbakk's conviction—then his acquittal—in the Birgitte Tengs murder investigation
On the Norwegian island of Karmøy in 1995, 17-year-old Birgitte Tengs was found raped and murdered. The crime would remain unsolved for nearly three decades, until advances in DNA technology and international genetic analysis reignited the investigation—only to expose profound weaknesses in how courts evaluate forensic evidence.
When Birgitte was killed, DNA technology was insufficient to identify her attacker with certainty. However, blood traces on her tights contained Y-chromosome DNA from the perpetrator. This evidence would sit in storage for years, waiting for science to catch up.
In autumn 2021, Norwegian police reached out to Mikkel Meyer Andersen, a Danish genetic expert, to assess the rarity of a Y-STR (Y-chromosome STR) profile recovered from the victim's clothing. Over subsequent months, four international laboratories conducted extensive analysis using advanced techniques including massively parallel sequencing. Genealogy studies traced the DNA profile across Norway and beyond. By early 2023, investigators believed they had their man: Johnny Vassbakk.
Birgitte Tengs Found Dead
17-year-old Birgitte Tengs is found strangled on Karmøy, Norway
Cousin Convicted
Birgitte Tengs' cousin is convicted of the murder and sentenced to 14 years in prison
Case Reopened
First reopening of the case with new DNA technology
Acquittal
Norway's Supreme Court acquits the cousin after seven years of imprisonment
SNP-DNA Breakthrough
New SNP profiling of old evidence identifies new suspect
In February 2023, Vassbakk was convicted of rape and murder. The Y-STR evidence had proven decisive—described in Norwegian as "tungen på vægtskålen," the tongue on the scales of justice. Yet from the beginning, there were complications. Y-STR evidence alone cannot stand alone in court. The DNA profile represented a "mixed sample," and prosecution arguments relied heavily on the genetic match as proof of Vassbakk's involvement.
The case might have ended there. But as the appeal process unfolded, new evidence emerged that offered alternative explanations for how the Y-DNA could have reached the victim's tights.
In December 2023, less than a year after his conviction, the Gulating Court of Appeal acquitted Vassbakk completely. The court's reasoning was damning: it identified systematic failures in the investigation and prosecution. Police and prosecutors, the court found, had operated under confirmation bias—once a suspect was identified, evidence was interpreted to fit that conclusion. The court criticized a "reverse burden of proof," where similarity was treated as proof of guilt rather than as one piece of potentially ambiguous .
Jan Helge Andersen Confesses
Childhood friend Jan Helge Andersen admits involvement after 28 years
Most significantly, the appeal court highlighted "investigative tunnel vision." Authorities had become so focused on the DNA match as proof of involvement that they failed to properly evaluate the broader forensic context. Modern forensic genetics requires what experts call "activity-level evaluation"—determining not just whether DNA matches, but whether the DNA's presence at a scene actually proves the person committed the crime, or whether innocent explanations exist.
The Birgitte Tengs case has become a landmark example of how cutting-edge DNA technology, combined with institutional bias, can lead to wrongful conviction. As of early 2025, the murder of Birgitte Tengs remains unsolved. No new perpetrator has been identified, and no alternative suspect has been charged.
The case has prompted broader reflection within Scandinavian and international criminal justice systems about how forensic genetics should be integrated into investigations and trials. Experts now emphasize that DNA evidence, however sophisticated, must be evaluated within a framework that considers alternative explanations and avoids confirmation bias.
For Birgitte's family, the acquittal means the search for her killer continues—28 years after her death.
Sources:
https://pubmed.ncbi.nlm.nih.gov/40393314/
https://nordjyske.dk/nyheder/krimi/17-aars-faengsel-nordjyde-hjaelper-norsk-politi-i-gammel-mordgaade/4115122
https://davidhorn.com/resource-hub/the-birgitte-tengs-case-a-continuing-saga-of-judicial-error/