I'm curious - did you call law enforcement? Not that they could or would do much other than take a report but you never know... stranger things have happened.
This has happened to my hunting party twice in the last 10 years in the Manashtash unit. The first time it happened to a friend on his first elk. 3rd day of the season he was hunting alone behind a gate. When he got to his truck with the liver & heart there were 4 guys in a red chevy that noticed that he had an elk down (blood, heart & liver gives it away). He BS'd with them for a few minutes before he left and came back to camp to get help. When we got back there, the drag marks were pretty clear in the trail and we knew what happened right away. After that happened, we always make notes of the license plates and descriptions of all vehicles we encounter. Never did find the douchebags but spent the rest of the week with our eyes open for them.

Last year, in the same spot, almost the same story but a different friend and this time we had thier license plate and a description of the vehicle. We called law enforcement. Turns out that they were locals. Later that evening LE found them with the spike hanging in thier carport skinned. Buddys tag was in the bed of thier truck. I'm not sure what the outcome was for the douchebags but i suspect nothing good.

Now I always keep a notepad of license plates and vehicle & person descriptions of everyone i encounter while hunting.
steve