"let's say we accept that the DNA evidence came from a third party. It would seem likely that there should be more of that DNA at the scene.
Where is it?
If the perpetrator was careless enough to not wear gloves while sexually assaulting JonBenét, should we not find many more of those skin cells on her shirt, on the blanket, on the ransom note, etc.?"
Imo, not necessarily. I can't begin to count the cases I've followed or I've watched concerning thousands of cases over the decades that resulted in only one or two fingerprints. Or they only found ONE tiny speck of blood belonging to the suspects etc. Yet time after time they found no further DNA or any type of forensic evidence throughout the entire crime scenes.
To this day it baffles me that David Westerfield hadn't ever been inside of the Van Dam home before in his entire life, but yet he entered in the dark of night climbing the stairs to the second floor where little Danielle's bedroom was located. He kidnapped her taking her out of her home. LE forensically tested the home from top to bottom yet found absolutley no forensic evidence he had ever been there. They didn't even find any trace evidence belonging to DW. No fibers or hair or fingerprints or footprints belonging to DW, the pedophile murderer of precious little Danielle.
And his case is not alone. Thank goodness they were able to find DVDs prints in his camper.
The last stats I read stated only around 65 percent of murder cases are solved due to the suspects leaving no trace of themselves behind OR the evidence left behind by the suspects doesnt match anyone in CODIS.
Imo, that is why we continue to see decades old murder cases finally solved due to being able to finally match the evidence police departments have had in their possession for years or decades, and were able to match it to the suspects whether through CODIS that is constantly being updated or through genealogy.
I did read due to LE and forensic departments being so overloaded, and shorthanded there are many thousands, and thousands of DNA profiles taken from violent crimes all across the nation.... that hasn't been uploaded into CODIS yet. States are being given grants to assist them in the tremendous backlog of entering DNA profiles from unknown offenders so they can be uploaded into CODIS.
Jmho