You'll notice that in most television crime dramas, the criminals always seem to confess under interrogation. This happens much more rarely than we would think.
Watch any one of the 28 different CSI programs and you would think that we are able to forensically prove and decipher every single criminal case, while everyday law enforcement does not have the resources, time or experience to conduct those types of investigations, not to mention that the physical evidence left behind is often compromised, incomplete or useless.
Law enforcement is a very inexact science. Media leads us to believe otherwise.