Mass Grave is one of the biggest mysteries in GTA SA. There were many theories about the killer, but I found out, that it could be Giorgio Forelli.
1st Argument: Ken Rosenberg, Giorgio's lawyer, moved to Fort Carson Medical Center to rehab. Giorgio could move with him. Also, Fort Carson has the connection to the road, that leads to the Mass Grave.
2nd Argument: He is very brutal, as seen in GTA VC i.e. Car Grave and possibility to be connected to Cement Shoe Men, so he could kill six people and dump their bodies in bags to a hole.
3rd Argument: He is the onliest Forelli Family member, that is alive, so he could live in 1992, the year GTA SA is set.
That's all what I can say about this. This is my first forum thread, so sorry for any mistakes.