Hello there
I watch a lot of forensic files type shows, dateline, etc. I sometimes see a "cold case" or long running case where a new suspect is found but the narrator/reporter says the DNA sample from evidence was "used up" and could no longer be compared to a suspect.
Example crime X occurs in 2009 and DNA evidence is collected from the victim. It is immediately run through CODIS and no match. Then a new suspect appears in 2012 and it is compared with their sample, no match. A 2016 suspect appears and again it is compared with their sample, no match. Finally, a 2021 suspect appears, but this time they are unable to compare due to no more DNA evidence sample being left, it has been all used up. Therefore, another method will have to be used to make a case
I don't understand this. DNA doesn't change. So once the original analysis was done in 2009*, why can't this be compared to the 2021 suspect? Is there a law saying a new analysis have to be done each time for each suspect?
I don't know if this is something you would even know, but i would guess you may have some tangential knowledge.
Thank you for your time
*(corrected "1997" to "2009")