I've always thought Jason, just due to their different temperaments and Jason being closer to Gangitano (he let whoever killed him into his house while dressed in boxer shorts and a robe) though I'm aware Mark is also a viable suspect. The evidence seems to suggest that The Munster wasn't complicit, but was aware of what was about to go down during his visit to the house that night.

Link to the particular forum you mention? I frequent a few myself. Robbo's site?