Thank you for your replies. We are not going to spend the money on seeing what killed it, but we have determined from the symptoms that they likely have Marek's disease and we are going to try to proceed accordingly. We read the article posted on this site about it, and it was very helpful...
I just ran into this thread doing a search, because we had 10 chicks, and all were doing well for 3-4 weeks, (except for one, who seemed to be smaller than the others and not developing her wings as fast). Then recently, within the last few days, 3 of them have died and two more are acting...