In the letter found by Mr. Adler, Hill’s sweetheart, Hilda Erickson, wrote that Hill had told her he had been shot by her former fiancé, Otto Appelquist — someone she had broken off with a week earlier and who had asked her “if I liked Joe better than him.” In her letter, she added, “I heard Joe tease Otto once that he was going to take me away from him.”

Historians say the letter is groundbreaking because it is apparently the first time anyone has stepped forward to explain exactly how and why Hill was shot. Neither Hill nor Ms. Erickson testified at his trial, although Hill did tell the doctor who treated his wound that a rival suitor had shot him.

The prosecution maintained that Hill had been shot by the grocer’s son, even though the police never found any bullet cartridges or traces of blood, other than the victims’, at the murder scene. Prosecutors used Hill’s silence to persuade jurors that he must have murdered the grocer.

Ms. Erickson wrote the letter in 1949 to Aubrey Haan, a professor who was researching a book on Hill. The book was never published, and Mr. Adler found the letter in papers stored in the professor’s daughter’s attic.

“When I first read the letter, it was a ‘holy cow’ moment because all these years people wondered about what happened that night,” Mr. Adler said in an interview.