Comment DNA comes in the first phase already (Score 2) 68
Right now, DNA comes in the first phase of the investigation in a lot of cases, especially in countries where the police can get DNA samples of large groups of people without a lot of paperwork and judges involved. Even if they need a judge for individual samples, they still use the characteristics to determine what ethnicity, eye colour and such the person that left their DNA at the crime scene has.
DNA almost always can't prove someone actually committed the crime, only that they were there at some point in time. If prosecutors are lucky, they can come up with some explanation why the DNA can only be left at the crime scene during or directly after the crime, but a lot of the time, that's not possible at all. Only in a very limited amount of cases DNA can be used to identify the perpetrator, mostly rape cases where semen was found and other evidence proves that it wasn't voluntary intercourse.
Really, DNA is rather useless unless you have a sample of a known criminal on file and you can match it to something that has to be related to the crime. Even then, it's common for criminals to contaminate crime scenes with "hair bombs" from barber shops or even items with traces on them stolen from a rival so they get implicated in a crime. Placing false DNA traces is much easier for criminals than placing fake fingerprints. DNA may serve as a tool to narrow down your search or find persons of interest but it seldom will do more than that.