NEW YORK — A man authorities say confessed to the infamous 1979 disappearance of a 6-year-old boy from his New York City neighborhood has been formally charged with murder and kidnapping, a major milestone in a case that has stymied investigators and Etan Patz’s devoted family for decades.

The indictment against Pedro Hernandez, 51, of Maple Shade, N.J., was made public Wednesday and sets up a potential showdown at trial over whether prosecutors can convince a jury that his claim that he strangled the boy — a secret kept for more than 30 years — is credible.

The suspect’s attorney has argued that Hernandez is mentally ill and prone to hallucinations, and his confession can’t be trusted.

“Nothing that occurs in the course of this trial will answer what actually happened to Etan Patz,” defense attorney Harvey Feinstein said in a statement. “The indictment is based solely on statements allegedly made by my client, who has, in the past, been repeatedly diagnosed as suffering from schizophrenia.”

Prosecutors countered that an exhaustive post-arrest investigation found enough evidence to seek an indictment and proceed to trial.

“We believe the evidence that Mr. Hernandez killed Etan Patz to be credible and persuasive, and that his statements are not the product of any mental illness,” said Erin M. Duggan, spokeswoman for District Attorney Cyrus Vance.

Etan’s disappearance led to an intensive search and spawned a movement to publicize cases of missing children. His photo was among the first put on milk cartons, and his case turned May 25 into National Missing Children’s Day.

The boy’s body has never been found. Etan’s parents, Stan and Julie Patz, have been reluctant to move or even change their phone number in case their son tried to reach out.

Etan was declared legally dead by his father more than a decade ago so he could sue convicted child molester Jose Ramos in the boy’s death. Ramos was found responsible, but it’s unclear how that finding could now factor into the prosecution of Hernandez.

Ramos, now 69, had been dating the boy’s baby sitter in 1979 and was considered a suspect. He was later convicted of molesting two different children and is in a Pennsylvania prison.

Investigators began focusing on Hernandez this year after a tipster called police about comments by Hernandez’s sister that she heard secondhand he told a church prayer group in the 1980s that he killed a child in New York City.

Hernandez, now a married father, was a teenage stock clerk at a convenience store when Etan disappeared on his way to school on May 25, 1979. Police say he told investigators he lured the boy into the convenience store with the promise of a soda.

He allegedly said he led the child to the basement, choked him and left his body in a bag of trash about a block away. The convenience store is now an eyeglass shop, and city records pinpointing where garbage was dumped don’t go back that far.

Following the arrest, court hearings for Hernandez were postponed for weeks on end, with both sides saying they were continuing to investigate. The prosecutor’s office said in September it wanted time to keep going “in a measured and fair manner.”

Authorities seized a computer and a piece of old-looking children’s clothing from Hernandez’s home, scoured the basement of the building where he had worked in what was then a grocery store and interviewed his relatives and friends — but nothing incriminating came of it, according to a person familiar with the investigation.

The person wasn’t authorized to discuss findings not yet made public and spoke to The Associated Press on condition of anonymity.

Psychiatric exams of the jailed Hernandez have found that he “has an IQ in the borderline-to-mild mental retardation range,” his lawyer said Wednesday.

Herndandez also “has been found to suffer from schizotypal personality disorder, which is characterized by, among other things, unusual perceptual experiences, commonly referred to as hallucinations,” he added.

The diagnosis could become the basis of psychiatric defense claiming that Hernandez agreed to speak to police without understanding his rights, and that the purported confession was a sick fantasy.