DNA Disasters on Dexter

This post is a reflection on an event that occurs in the first episode of the fourth season of Dexter. Currently available to watch here.

I'm not giving away any huge spoilers but if you don't want to know anything, or if you're the squeamish type, I wouldn't recommend reading on.

Okay, so the fourth season looks promising and I'm looking forward to its DVD release (no way I'm actually buying Showtime). But I have one bone to pick with the first episode.

Okay, so the new serial killer in town, the Trinity Killer, gets naked and kills a girl in her bathtub. Supposedly he cleans up after himself extraordinarily well. But they find the girl still in the tub floating in the bloody water.

Now here's what I want to know: since the killer was in the tub with her and naked, why couldn't they find his DNA? Did they simply not think to check the water? Seems a terrible oversight. It should have been a cesspool of DNA! If he was naked, all kinds of skin and hair could have been left behind. I suppose he could have let out the water he originally killed her in and filled the tub with new water, but this seems unlikely.

Anyway, perhaps I've just over-thought this whole thing...