NEW YORK (AFP) ― The man who this week confessed to killing six-year-old Etan Patz, who disappeared in New York 33 years ago, has left US justice with a massive dilemma: is the suspect guilty, or is he a fantasist?

Pedro Hernandez, 51, told police he strangled the boy after luring him "with the promise of a soda" to the basement of the grocery store where he worked, close to a school bus stop where the child was last seen in 1979.

He was charged with second-degree murder on Friday, with his arraignment in a Manhattan court done by videolink from Bellevue Hospital. He wore an orange prison jumpsuit and appeared dazed, with no discernible expression on his face.

Defense lawyer Harvey Fishbein has questioned whether Hernandez is mentally capable to face prosecution for Patz's death, a crime that rattled American parents and made them less keen to let children out of the house on their own.

The accused has "diagnoses of schizophrenia and bipolar disease. He's under medication and in fact was admitted to hospital because of the medication," and has "a history of hallucinations, both visual and auditory," said Fishbein.

Although police say Hernandez has made a lengthy confession to the crime, he has not yet entered a plea and officials have not said what evidence they have against him, beyond his confession.

Patz's disappearance shocked the nation and he was the first missing child to have his picture placed on milk cartons, appealing for information. May 25, the day that he vanished was named National Missing Children's Day.

Without Patz's remains, prosecutors have to rely on other evidence to build a strong case against Hernandez, who moved from the SoHo neighborhood where the child was last seen to neighboring New Jersey shortly after the mystery began.

But having chosen to press charges over such a long-running unsolved crime, the authorities have been circumspect in their remarks amid suggestions that Hernandez may have fabricated his story.

"This is the beginning of the legal process, not the end," District Attorney Cyrus Vance said in a statement. "There is much investigative and other work ahead, and it will be conducted in a measured and careful manner."