Garrison being such a coveted POI by Detective Thomas tells me someone of his ilk or even the fringe of him stood to lose out on a big money operation of sorts (likely drugs).
If Suzie was cross-examined in court and asked certain things about Joe or Dusty or Mike, it might shine light on something a certain person didn't want in the public domain. So silence needed to be ensured.
Why do you think Suzie had some inside drug knowledge? Or was being silenced She was just supposed to answer questions about the grave robbery. The GJ came about after they went missing. If someone was covering another crime up, making three women disappear was the most attention grabbing thing they could do. Suzie was back up testimony at best. Why wouldn't Joe Riedel be a target. If you are talking about the GJ it seems most people are alive. It doesn't make sense.