It’s a common trope in TV and movies: Cops wait until a suspect drinks from a glass or smokes a cigarette or spits, then collect their DNA to tie them to a crime scene. On the show Luther, however, John Luther went further—he punches a dude in the face and then collects his blood. Could a detective do that? The answer, unsurprisingly, is a hard no.
For one, punching someone is assault. For another, the evidence immediately becomes tainted. It can’t be used in any further prosecution. The evidence then becomes what law enforcement calls “fruits of a poisonous tree”—anything that comes from that tree is tainted.
“That’s a very interesting, and very illegal, way to get DNA from somebody,” says senior crime-scene analyst Matthew Steiner.
Instead of performing a face-punch, detectives usually get someone’s DNA through a court order or an “abandonment sample,” which are those left-behind cigarette butts and other bits of DNA that police collect when a suspect leaves them behind. Those pieces can be used in court—and they don’t require a right hook.
Want to know more dos and don’ts of crime scene investigation? Watch WIRED’s Technique Critique video with Steiner above.
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