Those are some good possibilities. Fingernails are often the source of tissue from a suspect, but not the only one. This killer wasn't averse to dismembering a victim. If Mr of Mrs. D scratched him, why didn't he remove the hands. That's assuming the tissue samples belong to the killer.
The article refers to "hundreds of fingerprints and hair and tissue samples". If the tissue samples belong to the perp, then do the fingerprints and hair samples, too? How do we know? And if so, doesn't that contradict the other facts we know about the crime? In other words, if the killer was as careful to cover his tracks as it seems, would he leave behind fingerprints?
Ach! Each question leads to ten more. Here's hoping for answers very soon!