The Devil's Mask (1946)

Based on the "I Love a Mystery" radio program (where most of the stories were told in serial fashion over several episodes): Searching through the wreckage of a crashed airline, the police find five shrunken human heads, and one of them is a white man with red hair. The head belonged to an explorer, deemed lost on an expedition to South America, and is tagged as a victim of jungle savagery. His daughter thinks otherwise, and suspects foul-play outside the jungle. She hires a team of private detectives to investigate, and points them in the direction of her stepmother and a colleague of her father, who appears to be more than a colleague of the stepmother. The detective, however, also looks in another direction and vocation for the killer.