DNA evidence can be fabricated - blood and saliva taken from one person can be made to appear as taken from someone else instead, say Israeli scientists.
'Today, DNA evidence is key to the conviction or exoneration of suspects of various types of crime, from theft to rape and murder,' the team wrote in their paper in the journal Forensic Science International: Genetics.
But the disturbing possibility is that DNA evidence can be faked. The team found current forensic procedures could not distinguish between their own artificial DNA and real DNA samples.
Lead author Dr Dan Frumkin said: 'If you can fake blood, saliva or any other tissue, you can engineer a crime scene. You have full control of the situation.'
Dr. Frumkin said: 'If you have some source material from the person who's DNA you want to fake, such as a hair, a glass they drunk from, or a cigarette butt, you could extract a minute piece of DNA using basic techniques you learn in your first year as an undergraduate in biology on equipment used in every biology lab.
'You can then produce billions of copies of it using very cheap kits that are commercially available. The process (known as whole genome amplification) is very, very easy if you have source DNA.'
The DNA can be applied to the surface of objects or could potentially be incorporated into human tissues and planted in crime scenes.
However, a fake sample can also be created just using DNA profiles stored in police databases. These are a series of numbers and letters that correspond to variations at 13 spots of a person's genome.
The scientists built up a library of more than 400 DNA snippets that represented common variants at each spot.
'This library contains all of the different variants found in the human population and you can create the desired profile,' Dr Frumkin said.
Dr Frumkin is founder of the company Nucleix, which has developed a new test to tell whether DNA samples are real or fake.