Jurors spent five full days weighing the evidence against Robert Eugene Pilcher, the Ottumwa Post reported. Judge Richard Meadows declared a mistrial Thursday afternoon.
Pilcher was arrested in 2012 and charged with killing Mary Jayne Jones. Her badly beaten body was found in 1974 in a farmhouse belonging to Pilcher's cousin, Max Marlin, who was in California at the time.
Prosecutors plan a second trial, scheduled for March 25.
Jones, 17, who had come to Iowa to visit a sister, decided to stay. She was living and working in Ottumwa when she was killed.
Investigators said she was beaten and then shot.
Pilcher, now 67, who had been a suspect earlier, was arrested in 2012 after investigators used DNA testing to link him to the crime. But the defense argued his DNA could have been at the scene because he had had sex with other women at his cousin's house.
Judith Cabanillas, Jones' sister, made a statement for the family, thanking prosecutors and investigators involved in the case.
"Although the jury in this matter could not come to a unanimous decision on the verdict in this trial, our family firmly believes that the defendant, Robert Pilcher, is guilty of the brutal and senseless murder," she said. "It is our hope that the county along with the Attorney General's Office will continue to pursue this matter and one day obtain justice for my sister, Mary Jayne Jones."
Pilcher remains in the Wapello County Jail.