They don't fit nicely into any single species, and yes, they are North American. But if I had to guess, I would say Lilium michiganense. They are not Lilium martagon. In the old days, some people referred to any lily that has petals that curve back in a turk's cap fashion as martagon lily. Thus, that would include most all of the North American species, as well as Lilium lancifolium, martagon, distichum, medeoloides, henryi, etc. This is probably where the error originated.