Two things. People used to be insanely superstitious and many still are.
Second, nowadays almost everyone has seen an X-Ray but they were still rare for the public in 1930. People only saw bones in cemeteries so it was associated with death. (When Röntgen took one of the first X-ray pictures of his wife’s hand, it’s said she fainted as it was like seeing her own corpse.)
Before fingerprinting the police had tried ways to extract confessions out of people, including a documented practice of standing outside a perp’s window in the dark and calling out that they were the ghost of the victim and why did you kill me?