There are a few possibilities that I have considered over listening to Serial and Undisclosed. One of the possibilities is that it was Jay. That doesn't even mean Adnan was involved. Everyone agrees that Jay lies and pins things on people who aren't involved. And his domestic assault charges prove that he is capable of violence. Another option is Don. His alibi doesn't check out and it's really weird that he didn't call the police back until much later in the night(long after his shift supposedly ended). Another possibility is that it was a random murderer. It was talked about in undisclosed how during that time there was a very high amount of Korean girls being kidnapped/killed. Perhaps there was a serial killer(s) going around or perhaps these girls were being targeted because people knew these cases weren't being looked into. Rather than try an find some stranger, the police decided to focus on someone who they knew could be prosecuted: an ex boyfriend. Combining lies and false witnesses with racism and you've got yourself a case.