A poster with the writing of accused Pedro Hernandez is pictured in this undated evidence handout photo provided by defense attorney Alice Fontier. Reuters

A retrial for a New York City boy who went missing 37 years ago is expected to begin this week. Justice has never been served for six-year-old Etan Patz, who disappeared while walking to a bus stop alone for the first time ever.

On May 25, 1979, Etan set out to walk the two blocks from his family’s home in the Manhattan neighborhood of SoHo to his school bus stop, the first time he was ever allowed to walk alone. It was the last time he was seen. Etan’s body was never found.

His disappearance changed the way kidnappings were investigated in the United States. Etan was one of the first missing children to appear on a milk carton. His family went on to push for new laws that created a missing children’s hotline and laws that made it easier for police to share information about missing children. The anniversary of his disappearance later became National Missing Children’s Day. The case prompted families across the country to forbid their kids to walk to school alone.

New York Police Department spokesman Paul Brown holds an original missing poster of Etan Patz during a news conference near a New York City apartment building, where police and FBI agents were searching a basement for clues in the boy's 1979 disappearance, in New York April 19, 2012. Reuters

It took 33 years for a suspect to be arrested for Etan’s disappearance. In 2012, police arrested Pedro Hernandez, who worked in a convenience store near the family’s Prince Street home. Hernandez’s brother told police that he had mentioned killing a boy to a prayer group in the summer of 1979.

Police proceeded to extract a confession from Hernandez that would prove fraught with complications. He told police he lured the boy into the store with the promise of soda, where he then choked the boy before leaving him in a back alley. But no physical evidence linked the man to the murder or kidnapping of Etan Patz. His defense attorney argued that his confession was a delusion caused by mental illness. A psychiatrist testified that Hernandez’ schizotypal personality disorder could induce this type of delusion. His daughter told of her father’s hallucinations, explaining that he talked to himself and saw things that weren’t there.

Pedro Hernandez appears with his lawyer Harvey Fishbein (unseen) in the Manhattan Criminal Court in New York Nov. 15, 2012. Reuters

Hernandez’s lawyer urged the jury to consider, instead, a man named Jose Ramos. Ramos was a convicted child molester and the original suspect in the case. He had dated a woman who occasionally walked Etan home from school.

Hernandez’s trial ended in May 2015 after 15 days of deliberation. The jury couldn’t come to a unanimous decision about his guilt. With one juror holdout, it ended in a mistrial.

The opening statements in the new trial are set to begin sometime this week. No body and no physical evidence made it a difficult case the first time around. Since so much time has now elapsed, the difficulties this time around may prove even greater.