How did the DNA of a suspect get to the crime scene? A practical study in DNA transfer during lock-picking

Increasingly in courts, the source of forensic DNA trace evidence is not challenged by the defence. Instead, the issue relates to how the suspect’s DNA got to the crime scene, suggesting it arrived via a legitimate activity. Many burglaries are committed using a new lock-picking method called ‘foil impressioning’, which involves inserting aluminium foil into the lock. When DNA is recovered from this foil, suspects will say that they only cut the foil and sell or discard it, to be used later in lock-picking by someone else. This study elucidates the probabilities of recovering DNA profiles from aluminium foil in three situations: (1) when a person only cuts the foil, (2) when they cut and then use it for lock-picking and (3) when they cut it and someone else uses it for lock-picking. Using a Bayesian network, we conclude that the probability of recovering a person’s DNA profile from foil inside a picked lock is seven times higher if they picked the lock than if they only cut the foil. This study demonstrates how likelihood ratios based on probabilistic models of DNA transfer and persistence can now help the court address questions about the mode of deposition of DNA traces.