Since the Hispanic websites assign the Quechuan and Castillian lyrics to Robles, I accept that. Certainly none of the Americans who took up the tune were capable of writing them.

Robles studied botany and zoology, was trained as a doctor, but abandoned that career for one in music. He was considered an authority on Andean folk music; as noted above he made collections in Andean folklore and poems, as well as composing some 146 classical and popular pieces of music.
His son, the film producer, known for his classic "La Muralla Verde" (The Green Wall), has collected his works. I don't know how to access these papers and MSS.

I don't usually accept Wikipedia as an authority; their article, based on the Spanish Wikipedia (linked above on my third try), cites Robles as the composer, "based on traditional Andean folk tunes."
The English article, much abbreviated, here:
http://en.wikipedia.org/wiki/El_C%C3%B3ndor_Pasa