I believe police worked up a deal with Jay. I think Jay was much more involved and potentially the one who killed her in order to frame Adnan out of his own jealousy surrounding Adnan and his close relationship with Jay's gf Stephanie. Adnan was an athlete and star student. Jay was a high school drop out who sold weed. Stephanie was Jay's only saving grace and Adnan threatened that for him. Adnan even mentions how he and Stephanie were very close and that she'd sit on his lap and play with his hair in school which is enough to make any teenaged boy jealous. This is Jay's motive. I think police doctored Jay's story and fed him what to say in exchange for excusing him on any potential drug related charges and to have someone to pin the blame on and make it seem like they legitimately solved the case. Jay became an informant. There's no DNA linking Syed to the crime.