Imagine that a convicted child molester is released from
jail and
that he is now traveling around the country looking for work. One day
this criminal returns to his old ways and he attacks a young child whose
dead body is found in a deserted field the following day. The only trace
of evidence at the crime scene is a semen specimen on the boys
clothes.
Now this specimen could be useful if the police tracked down, and arrested
the suspect, and then took a blood sample of his that matched the crime
scene specimen. But, since no other clues are found this criminal was
never arrested and for the next couple years he repeated this act many
more times.
