When an absent investigator’s DNA turns up on a sealed exhibit

A case that had gone unsolved for eighteen years is reopened. From a plastic bag kept as a sealed exhibit, analysts obtain a full DNA profile. Run against the staff elimination database, it returns a name, that of an investigator. The trouble is that this investigator was eighty kilometres from the laboratory and had never set foot at the scene or in the exhibit storage room. The profile was nonetheless his. DNA had identified a person with near-certainty on an item he had never handled. Reported in early 2026 by Gao et al. in Forensic Science International: Genetics, this case is a reminder of a distinction that is too often overlooked: knowing whose DNA a sample carries and knowing how it got there are two different questions.

Identifying the source is not explaining the activity

The forensic laboratory’s result answered one question perfectly, the origin of the genetic material, but it offered no reliable information about the circumstances of deposition. This distinction has long shaped the reasoning of forensic geneticists. In 1998, Cook, Evett et al. [1], writing in Science & Justice, proposed a hierarchy of propositions setting out three levels of interpretation, running from the source to the offence itself. The sub-source level, specific to DNA, was added later, notably by Evett et al. in 2002 [2]. Establishing whose biological profile a sample matches sits at the very bottom of this ladder. Explaining how that profile came to be there, on a substrate or at a crime scene, belongs to the activity level, which is higher and far harder to establish. For a judge, the distinction is decisive. A laboratory can state with great confidence that the DNA profile obtained from a biological sample matches an individual’s profile, yet cannot say, with the same assurance, what that presence means.

The four levels of interpretation: a textbook case

The classic Cook and Evett example, a sexual assault case, where each level adds a further layer of inference on top of the one below:

• Sub-source level:
  “The DNA in the sample comes from the suspect” (rather than from an unknown person). This is what the raw DNA comparison measures directly. It speaks only to DNA, nothing more.
• Source level:
  “The semen comes from the suspect.” This adds identification of the body fluid, provided it has been established that the stain really is semen.
• Activity level:
  “The suspect had sexual intercourse with the complainant.” This adds the mechanism of deposition where transfer, persistence and contamination come into play.
• Offence level:
  “The suspect raped the complainant.” This adds the absence of consent which the expert cannot decide, since that is for the court.

At each step up, science can say a little less, and the weight of the evidence diminishes.

How more sensitive methods changed the picture

The problem is not new, but it has changed in scale as DNA analysis has advanced. Today’s kits can recover usable DNA profiles from just a few cells, where once a far larger amount of DNA was needed. In 1997, van Oorschot and Jones [9] showed that mere contact is enough to deposit DNA. Since then, the literature has documented many routes of transfer, both direct and indirect, surveyed in van Oorschot et al.’s 2019 review [10]. The effect cuts both ways. The very sensitivity that makes it possible to reopen old cases also raises the chance of detecting a DNA profile unrelated to the events in question. Studies remain cautious about frequencies, which vary widely with conditions, so much so that some authors regard secondary transfer as an uncommon event. Generalisation must therefore be handled with care, in both directions.

The risk of DNA transfer in evidence handling

The journey of a sealed exhibit, from the crime scene to the laboratory bench, passes through a series of handling steps during which DNA can move. The packaging of the item itself plays a part. Stella et al. [7] showed in 2026 that this transfer can run two ways: DNA deposited on an item may migrate to the inner surface of the packaging, so that the sought-after profile is missing from the analysis; or it may transfer to another area of the item, where it has no business being and clouds interpretation. Goray et al. [6] noted in 2019 that the outer surface of gloves could carry, alongside the target profile, that of the examiners themselves, particularly when opening and resealing the packaging. It is precisely this risk that the examination protocols used by forensic services are designed to head off, requiring decontaminated gloves changed for each new item. Examination tools pose a comparable problem, described as early as 2015 by Szkuta et al. [8]. That an investigator might contaminate an item or surface after the fact is therefore no far-fetched hypothesis; Fonneløp’s team [3,4] has studied the phenomenon right down to evidence bags themselves.

To explain the presence of this profile on an exhibit the investigator had never come near, Gao et al. [5] rule out item-to-item transfer in favour of a likely airborne route: the DNA is thought to have settled on a colleague’s clothing before detaching as that colleague approached the sample pre-treatment area. Without the staff elimination database, this hypothesis would never have come to light.

What this distinction changes for the evaluation of evidence

For legal professionals, the lesson of this case is not that DNA is unreliable, but rather that a solid genetic identification at the source level can coexist with a fragile explanation at the activity level. The whole risk lies in sliding from one to the other, when the strength of the former colours the assessment of the latter. To guard against this slide, some forensic geneticists argue that the expert should reason explicitly at the activity level and set out the scenarios consistent with the observations. This is notably the position of the School of Criminal Justice at the University of Lausanne and of the European guideline on evaluative reporting [11].

Conclusion :

The case of the investigator who was never at the scene illustrates the stakes better than any theoretical reminder could. His DNA profile, entirely genuine, sat on a sealed exhibit he had never touched, in a case whose scene he had never visited. Without a safeguard that brought another explanation to light, this finding could have been read as a genuine investigative lead backed by a “relevant” DNA profile. A genetic result is worth not only what it identifies, but what it reasonably allows one to infer, and that is never settled on a single trace alone. A DNA profile must be weighed within the wider body of evidence, never in isolation.

References :

• [1] Cook R., Evett I.W., Jackson G., Jones P.J., Lambert J.A. (1998). A hierarchy of propositions: deciding which level to address in casework. Science & Justice, 38(4), 231-239.
• [2] Evett I.W., Gill P.D., Jackson G., Whitaker J., Champod C. (2002). Interpreting small quantities of DNA: the hierarchy of propositions and the use of Bayesian networks. Journal of Forensic Sciences, 47(3), 520-530.
• [3] Fonneløp A.E., Egeland T., Gill P. (2015). Secondary and subsequent DNA transfer during criminal investigation. Forensic Science International: Genetics, 17, 155-162.
• [4] Fonneløp A.E., Johannessen H., Egeland T., Gill P. (2016). Contamination during criminal investigation: Detecting police contamination and secondary DNA transfer from evidence bags. Forensic Science International: Genetics, 23, 121-129.
• [5] Gao L. et al. (2026). A rare and atypical case of long-distance indirect DNA transfer: Contamination from an investigator never present at the scene. Forensic Science International: Genetics.
• [6] Goray M., Pirie E., van Oorschot R.A.H. (2019). DNA transfer: DNA acquired by gloves during casework examinations. Forensic Science International: Genetics, 38, 167-174.
• [7] Stella C.J., Goray M., Meakin G.E., van Oorschot R.A.H. (2026). DNA transfer in packaging: Investigation of mitigation strategies. Journal of Forensic Sciences, 71(1), 197-210.
• [8] Szkuta B., Harvey M.L., Ballantyne K.N., van Oorschot R.A.H. (2015). DNA transfer by examination tools, a risk for forensic casework? Forensic Science International: Genetics, 16, 246-254.
• [9] van Oorschot R.A.H., Jones M.K. (1997). DNA fingerprints from fingerprints. Nature, 387, 767.
• [10] van Oorschot R.A.H., Szkuta B., Meakin G.E., Kokshoorn B., Goray M. (2019). DNA transfer in forensic science: A review. Forensic Science International: Genetics, 38, 140-166.
• [11] ENFSI, Guideline for Evaluative Reporting in Forensic Science, 2015