I think:
The shirt was put there by JM.
I don't think it came to be there by someone "connected" or "with knowledge," or by family or friends. I still think someone may be out there who either participated in some way or has direct knowledge, (even after the fact), but why would they place the shirt there? Obviously a "cohort," by seeking to implicate JM, also risks implicating himself. If they were only extraneously involved and felt guilt enough to want to get the truth out there, why not place the shirt near JM's residence or somewhere more connected with him? Same thing for friends and family....if they could not bring themselves to go to LE, but felt they had to anonymously leave a clue for them, why place the shirt there of all places? I also don't think JM would ever allow someone else to SEE the shirt, even in the off-chance that he broke down and told a family member or friend about the incident, (which I really don't think he did.). I think he compartmentalized his bad deeds and no one was privy to them, with the possible exception of another bad guy who may have been somehow involved.