Reading page 301-303 of the Massei report is like reading fiction. The source of DNA could be established, it's just that the "when" should remain a question - and stunningly, Massei makes that very point and then proceeds to ignore it.
Why? Because Amanda herself had said that the bathroom had been clean prior to the murder.
But not just any clean - Massei assumes this observation on Knox's part rendered the bathroom forensically sterile, ready to receive forensic material which could only be dated/timed to the night of the murder.
In a bathroom the two of them shared.
Equally stunningly, Massei assumes that Amanda had stepped in the victim's blood. Yet, there is no evidence at all of Amanda's foottrack in the murderroom itself!
And without stretching this out too long - it is painful reading Massei's reasoning here... read how Massei justifies not ruling out blood, even though blood had not been found!
Instead of proving that blood had been there, Massei assumes there was and leaves it up to someone else to prove that there hadn't been any. Absence of evidence is not evidence of absence.
That, folks, was the 2009 case against RS and AK.
But Stefanoni had not said that it was impossible to determine the source of DNA, but.....
Although it is safe to say that the blood was the victim's because Massei says that Amanda had not been wounded - at all.