Tracking down a chain letter scam

Tweet

So I got in the mail today, a chain letter. One of these scams that says to send a dollar to the top person on the list, and forward it on to X people, and then put you name on it. blah blah blah. My question is this. I would like to send a rather unflattering letter back to the person that sent it to me. The problem is that there are six people on this list, and I don't really know if the last person on it sent it to me. Does anyone know if there is a way to be sure who sent it? I don't want to go making random people angry after all.