I am not positive who killed Shannan, however I am pretty confident MP knows who, what, where, when, why and how.
Dr. Hackett had very close ties with the SCPD, and had access to the gate camera video, and actually met and treated Shannan that night. The 911 call was with-held in my opinion, to protect the guilty.
Even though the SCPD was not actively investigating anybody in the case for all those years, they claimed they were.
If you want to cover up a crime, hide or destroy 'all' the evidence you can. 'Leave no loose ends' is the term I believe