For the sake of conversation, assume Adnan is factually innocent and completely uninvolved in the crime. Let's assume what seems to be the leading innocent Adnan theory, that Jay was uninvolved, and cops used Jay to pin the crime on Adnan.
For brevity, on this theory, the homicide cops become aware of Jay and start pressuring him at some point before his first "official" interview by leveraging his and his family's drug dealing over him. Presumably during this time, they also start crafting the narrative they will use with Jay during his interviews, locate the car that they will bring Jay to but make it appear Jay has know about and bring them to, etc. Jen also becomes involved during this