I can't quote exactly, but I believe LE stated something to the effect that they know "what happened" to her that night. I think if suicide was still on the table, LE may have stated something more like...we know what JC did that night. The other thing that throws suicide out, IMO, is that they are confident someone was with her. The car seats being reclined (both driver and passenger) seems to point to that as well, though not necessarily.
I know suicide by fire has happened, and will happen...but I don't see it as likely. Especially since guns are probably easy to acquire and pills as well.