>> it was the piece of evidence they were looking for for 16, 17 months. >> reporter: it tied marni yang scene. the case was not closed. prosecutors wanted more. they wanted to get marni talking about the murder on tape. because she was so guarded on the phone, christy agreed to wear a wire and meet marni at the usual dining spot. christy, like shaun was an unofficial agent. she had ideas on how to trap her best friend. >> she knew how her mind worked. >> reporter: what was the approach? >> we had information to link both of them to the scene. >> reporter: marni thought she was confiding but she was giving police an astounding confession. she told how she put it in a bucket of cement and threw it in the dumpster and explained what she wore the morning of the murder. >> a wig. dark sunglasses covering my face. i had dark make up on my face and i had gloves on. >> reporter: just like the mysterious person the neighbors saw. then she whispered details only the murderer would know. >> she started screaming. i took, i took the first shot. i remember screaming cause at that point i realized we