You were the one who brought up the fat suit initially Maria, which means that fits with the letter we found especially if it's forged. You were also the prime target for the Spider woman since you'd already shown the capability to luminol, finger print, etc. I think using those chemicals you either placed a trap in or around your vent and waited, possibly something that would have exploded. Ryoko tries to get rid of you and your lab, but she's caught up in whatever you've planned and as a result is killed, probably a small explosion. It explains the noise I heard, and the bullet likely was thrown across the vent from the force and fell down into my room. You were also in the perfect position to cover your tracks since you could dictate what you investigate and what you don't. I think you've played a terrific game, and even if I'm wrong about Dexter killing Biscuit I'll make sure I go down having gotten you.