Dna tracking down a killer question?

I'm writing a short story and I'm curious, how does the fbi track rapists? Say the rapist leaves traces of sperm on the victim, will the FBI be able to track the guy down or would he have to have submitted a sperm sample in the past to be tracked down?



Does the FBI take sperm samples? I heard that they take a cotton swab and rub it against the persons mouth, can they compare that to the sperm sample to track the guy down?



And are there any jobs that require you to submit dna to get the job?



I want to come up with an accurate way to get the killer caught in my story.Dna tracking down a killer question?
The FBI wouldn't generally get involved in rape or murder cases -- these would fall to local authorities unless the crimes were committed on federal property. I don't know of any law enforcement organization that takes sperm samples from males -- a cheek swab is all that is needed.