For years banks have rigged bags of money with exploding dye packs, which show the cash was stolen and mark the thief. Now DNA can do the same job -- without the suspect being aware of it.
This isn't using the criminal's own DNA to track him or her -- it's engineered, artificial gene sequences that act like bar codes. They can be applied to goods or people to uniquely identify them, and be made to glow under certain kinds of light or be read by swabbing them and reading the sequence chemically.
DNA marking is already being used on objects for tracking by law enforcement agencies in the United States and the U.K.
The latest version of the technology comes from Stony Brook, N.Y.-based Applied DNA Sciences. It's called "DNA Fog." The device fills a room with smoke to confuse an intruder. The smoke isn't just to make it hard for the person to see; it also contains droplets loaded with DNA. If the person escapes, they are still covered with it, and it's invisible.
The DNA stays on the skin for about two weeks and is hard to wash out of clothing. And even if the burglar ditches her clothes in a dumpster, she'd have to lose the shoes, too.
"It's amazing how many people will dump their clothes but not their shoes," said Mitchell Warren Miller, director of digital strategy at Applied DNA. The "tag" of DNA lasts about two weeks.
Should that person get arrested, police would swab them and read the sample using a chemical process called polymerase chain reaction. PCR amplifies the genetic material, essentially making it easier to spot, and is simple and cheap enough that it can be done by amateurs. If the sequence from Applied DNA shows up, then it's a good bet they were in the vicinity when the DNA Fog device went off.
The DNA sequence can be altered, so any business that has it can have it's own unique code -- like tagging an intruder with the name and address.
It's an adaptation of a system Applied DNA uses to track goods, by marking them with DNA that glows when exposed to certain kinds of light, except in this case the DNA marks a person invisibly.
In the U.K., Selectamark Security Systems' makes several devices to get a DNA tag onto someone. One is a "defense spray" like pepper spray, which is geared to security guards and police who might want to identify someone who attacked them at close range. Another is a grease and gel that mark door handles or goods.
But perhaps the most sophisticated DNA tagging product is the "high velocity DNA Tagging system" that works as a kind of paintball gun. The gun uses pellets filled with water, DNA and a binding agent to help it stick. The pellet is a polymer that differs from the kind in paintballs, as it has to be strong enough to be jostled around, weak enough to break on impact and most importantly not react with the DNA.
SelectaMark currently testing out its pellet gun with several police departments in England, and it will be some months before they decide whether to adopt it.
DNA's unique, invisible properties are being harnessed to fight crime. Wavebreak Media/Thinkstock
Built for riot control, the gun is deigned to identify people police might want to question afterwards. Andrew Knights, managing director of Selectmark, said the likely scenario is when police see someone they think instigated a riot.
In Britain, it isn't uncommon to have "snatch squads" go after specific people during a disturbance, but that can escalate the situation. Instead, they'd fire the pellet at the person they are interested in. Once marked, he or she might not see anything as the water will evaporate. But as soon as the mark is exposed to ultraviolet light, it glows.
The mark doesn't just tell officers that the person was hit with the gun. It can also tell them exactly where she was and when. That's because the pellets in each gun have a specific DNA sequence, matched to the officer it is issued to. Since the firing is authorized by a superior, it's easy to tell when the suspect picked up the mark.
Combined with evidence from local video cameras and witnesses it provides an ironclad case for when a given person was at the scene of a crime, said Knights.
"It closes the evidentiary triangle," Knights said, referring to the time, place, and the person involved in an incident. "They can't say 'it wasn't me, just some bloke who looks like me.'"
The two companies use slightly different types of DNA. Applied DNA uses a two-stranded set of sequences derived from a plant, though the sequence itself is completely artificial. SelectaMark uses single-stranded DNA. (Neither set can reproduce, nor are they functional).
Such tags are hard to copy. DNA is made of four nucleotides, which make up the code that geneticists read. In living things those nucleotides (marked A, G, C, and T) that make base pairs, and they can only occur in certain combinations because the DNA has to code for proteins. But artificial DNA doesn't have that constraint. A single strand of DNA just 20 base pairs long gives 1.1 trillion possible combinations.
It might be possible to fake such sequences, but only if one knew the sequence to begin with and had access to a pretty sophisticated biology laboratory. On top of that, it's easy to insert "decoy" DNA that isn't part of the code sequence. (Imagine coding a message and spreading random letters in it – unless you knew the message at the outset it would be hard to decode).
Miller said the focus on identifying people this way comes, partly, because many anti-theft devices aren't very good at helping to catch people. "Precisely because many products are good at repelling people, they aren't very good at apprehending them," he said.