"Circumstantial evidence is evidence in which an inference is required to connect it to a conclusion of fact."
"Forensic evidence supplied by an expert witness is usually circumstantial evidence."
Exactly, there's where the weakness is regarding both the clasp and the knife. Even before independent review it was clear that the evidence is invalid, because no inference to connect those findings to the crime can be reasonably made.
With this in mind, what are the judges expecting in a violent assault and murder- pristine, ideal, sterile lab conditions or are they expecting a mess and a search for clues amidst this contaminated mess?
What was definitely preventable (if not unexpected) was the mess that have been created after the crime by the police trashing and trampling the scene and by scientifica geniuses giving a group massage with dirty gloves to that bra clasp.
Things happened as they did, inside a not a pretty or scientifically controlled picture, or under rigorous lab conditions. The key in a case like this is the different elements found by investigators and later testimony in court, including corroborating experts, working to a coherent narrative that explains the victim's death and who is responsible.
Again, spot on! There is no coherent narrative for the knife, when you have to concede that the wounds and the bloody imprint were done with another blade, when you have to introduce crazy theories to explain how the knife got there at all and why and how it got back to the drawer with starch where the blood was expected.
There's even less chance to piece together a story for the clasp - when the only DNA on the bra itself belonged to the victim and Guede, when there is not a single trace of anyone else there and heaps of evidence for Guede's presence. More so, you can't come up with anything resembling a coherent narrative putting the two together with Guede there. You cannot even place AK and RS there because they have alibi for the only reasonable ToD.
Contamination is to be expected in a violent assault and murder.
Contamination is also to be expected when cops trash the scene for a month and a half, and then pull out circus with walking a gift wrapped mop around and fondling the evidence wit their dirty fingers. So if we add to it the expected contamination from before and during the crime, what is it that is left?
The problem for Sollecito is just how it fits within the narrative. Is it completely unexpected that Sollecito's traces appear on the clasp or not?
Cynics will say that it is completely expected, given that he's already arrested with a "caso chiuso" triumphantly announced and suddenly the only piece of evidence against him is no longer there, prompting a desperate excursion extraordinaire to get something new to keep him in jail.
Fortunately for all, contamination with his DNA
is also expected and cannot be excluded during every stage of the forensic cabaret that we witnessed from the simple fact that the cottage, the lab and the questura had plenty of his DNA throughout those 46 days the clasp "went missing".