From the book The Hammered Dulcimer: A History by Paul Gifford, p. 25:
"Iconographical and textual evidence contradict the widely influential writings of Curt Sachs, which claim that the dulcimer's origin was in the Middle East. Subsequent articles have repeated his claim so much that it popularly has become fact. However, Sachs confuses the psaltery with the dulcimer in using the relief sculpture at the cathedral of Santiago de Compostela as evidence and provides no further source for his claim. Chapter 4 demonstrates that in Iran the santur first appears in its modern form as a dulcimer only at the end of the fifteenth century and that in an earlier, Egyptian form it was a vertically held box-zither identical to the early qanun. The dulcimer, on the other hand, first appears in textual and iconographic sources from Western Europe during the early fifteenth century, prior to the earliest Islamic iconographic evidence of a struck dulcimer. Had it arrived in Europe from the Islamic world as a fully developed dulcimer, we would expect to find earlier evidence of the instrument. None, however, has surfaced. Therefore we must accept the thesis, argued by both David Kettlewell and Herbert Heyde, that the dulcimer arose in Western Europe independently."
More information can be obtained by reading Paul Gifford's complete book as well as reading David Kettlewell's online PHD Dissertation on the Hammered Dulcimer with notes added in 2002
Nancy Groce's book The Hammered Dulcimer in America originally published by the Smithsonian is now available for download here.
You can also access more information through my Hammered Dulcimer Resources page.
I occasionally post photos of historical hammered dulcimers I run across to a board on Pinterest. You are welcome to add to that board.