After crossing Ohio, Indiana, and Illinois as part of Zion’s Camp in 1834, Joseph wrote a letter to his wife Emma. He described “wandering over the plains of the Nephites, recounting occasionally the history of the Book of Mormon, roving over the mounds of that once beloved people of the Lord, picking up their skulls & their bones, as proof of its divine authenticity.“
Where are the plains of the Nephites? The Book of Mormon describes several plains where events took place, including plains near the city of Mulek (Alma 52:20), the plains of Agosh (Ether 14:15), the plains of Heshlon (Ether 13:28), and the plains of Nephihah (Alma 62:18). Joseph could have been referring to any or all of these.
As recorded in the Doctrine and Covenants, the Lord sent missionaries to the Indians living in New York, Ohio, and Missouri, specifically identifying them as Lamanites and telling them that He (the Lord) “would go with them and be in their midst.” (D&C 32:2) During this mission, Joseph Smith told tribes from Michigan that the Book of Mormon was the history of their ancestors. He wrote that the Book of Mormon is a record of “the forefathers of our western tribes of Indians.” 2
The Firm Foundation. The Smoking Gun of Mormon Geography. By Rod Meldrum. http://firmlds.org/smoking-gun-book-mormon-geography/#note-54-2
Just to drive the point home — Joseph Smith sent missionaries to North American natives to teach them of their Book of Mormon ancestors. If hindsight provides 20/20 vision, the question of where the Book of Mormon people lived is North America. Could they be the Mound Builders?