Christina Morales ? (spoilers)
I think its HER BLOOD on that belt!
She is the familiar DNA link because they are all her half siblings!
She killed her own father (the last vic)!
and when the police officer told her on the phone that Garrett was the suspected Silver Bells Killer before Garrett got to tell her anything (possibly that he knew she killed her own father somehow and that its ok) -------- she saw her opportunity to KILL HIM and have everyone think case closed and she would get away with it!
Could this all be the real reason he has been watching her all these years and documenting everything in his book about her?
I am going to go back and rewatch all 6 tonight and pay closer attention to her LOL