From Handbook of American Folklore by Richard Mercer Dorson (Indiana University Press, 1986), p.414:

For example, the ballad "The Last Fierce Charge" (Laws A17) can be traced to a poem by Virginia Francis Townsend.

The footnoted reference is D. K. Wilgus, notes to "Native American Ballads," RCA Victor LPV-548. Dorson makes this statement as an example of a ballad being traced to a specific original source.

The Townsend attribution is guardedly repeated in Folk Songs of the Catskills, Norman Cazden and others, p.79, with a different reference, though probably pointing to the same bit of research.