goes dead and nobody ever heard from her again. >> it was 3:42 in the morning. police now had a theory, somehow, heather had become a problem for sydney so he lured heather to the landing and killed her. and there was more, investigators believed sydney and heather weren't alone. someone else was at the landing. >> coming up -- who was that mystery pursuing at the landing? and why would anyone want to harm has her? >> the fire, the jealousy explodes into utter rage. >> when "dateline" continues.