Jack Starrett replaced original director Lee Frost when Twentieth Century Fox head Alan Ladd Jr. was unsatisfied with the dailies. Most of his footage was re-shot.
The picture of a human sacrifice that the women find in the library book is from a series of images of Aztec Indian life painted by H. M. Herget for the June, 1937 issue of National Geographic magazine.
Coincidentally, Peter Fonda's screen wife Lara Parker once roomed with his sister Jane Fonda at Vassar College, Poughkeepsie, NY.
Parts of the ominous opening theme sound like The Exorcist, which had started a wave of Satanic Cult movies after the muse of Rosemary's Baby had worn off.