New York (CNN) -- A lawyer for a man accused of killing 6-year-old Etan Patz in 1979 says his client falsely confessed to authorities in May and will plead not guilty when he is arraigned next month.
The attorney's statement Thursday morning came after Pedro Hernandez made a brief appearance in Manhattan criminal court.
"The statements by my client (to police last spring) are not reliable," defense attorney Harvey Fishbein said. "They are what we term false confessions."
A grand jury on Wednesday indicted New Jersey resident Hernandez on charges of second-degree murder first-degree kidnapping. At Thursday's hearing, a judge transferred the case to New York State Supreme Court -- the state's main trial court system -- in Manhattan and set an arraignment for December 12.
Police said Hernandez in May confessed to killing Patz, who disappeared on his way to a New York school bus stop more than 33 years ago. Hernandez -- who was a stock clerk in Lower Manhattan in May 1979 -- admitted that he choked the boy after luring him into the basement of a Manhattan grocery store, police said.
Hernandez allegedly told authorities that he threw away the boy's body in a garbage bag. The remains have not been found.
Etan disappeared on May 25, roughly a month before Hernandez left New York to resume living at his mother's south Jersey home, according to family members and police.
Fishbein has claimed Hernandez has been repeatedly diagnosed with schizophrenia, and that he has "an IQ in the borderline-to-mild mental retardation range."
On Wednesday, district attorney spokeswoman Erin Duggan said the indictment was "the outcome of a lengthy and deliberative process, involving months of factual investigation and legal analysis."
"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," she said Wednesday.
Besides arguing that the confession was false, Fishbein said he would argue that the prosecution has insufficient evidence, and that the statements Hernandez made to police are inadmissible.
Patz's plight catapulted concern for missing children to the national forefront after authorities put his image on thousands of milk cartons, a technique that would become more common in the next few years.
CNN's Ross Levitt and Jason Hanna contributed to this report.