For more about the disappearance of 6-year-old Etan Patz, a case that frustrated and mystified police for decades,read the story ProPublica produced with WNYC in 2013.

Stan Patz, back straight, betraying no outward emotion, sat in a Manhattan courtroom on Monday and for the first time listened to someone confessing to killing his 6-year-old son Etan.

Pedro Hernandez, a former bodega clerk in the SoHo neighborhood where the Patz family lived in 1979, said he randomly encountered Etan on the street one morning as he made his way to the store's basement. Hernandez, in a taped confession made in May of 2012, said he had never seen the boy before. He asked the boy if he wanted a soda, unlocked the gate to the basement and had Etan follow him down the steps. Without either of them saying a word, Hernandez said, he grabbed the boy around the neck and choked him.

"Something just took over me," Hernandez told a prosecutor in the confession, which was played Monday in court.

"I wanted to let him go," he said. "I just couldn't."

When he lay the boy on the basement floor, Hernandez said, the boy appeared to still be alive, his body twitching or squirming. Hernandez said he put the boy in a garbage bag, and then inside a box that might have once held bananas. He hoisted the box on his shoulder, climbed the basement stairs, and carried the box several blocks, where he placed it in an alley.

"That's it," Hernandez said.

Hernandez, 18 in 1979, said he then returned to the bodega and had an unremarkable day of work: making sandwiches, stocking soda and beer, and saying nothing.

"My mind was blank," Hernandez told the prosecutor. "That's how I was feeling. I just feel like nothing. I didn't do nothing wrong."

The case of Etan Patz captivated and horrified New York City 35 years ago: the boy had disappeared on his first day of going to school alone. One of the most prolonged and intense police manhunts followed in the days and years after, and suspects surfaced and faded. But the boy's body was never found, no one was ever criminally charged, and thus Stan Patz and his wife Julie have lived with an aching uncertainty for decades.

Whether that uncertainty will be ended by Hernandez's confession was the subject of the courtroom hearing Monday. Hernandez's lawyer has claimed his client is a deeply mentally ill man who had been manipulated by detectives into confessing to a crime he didn't commit. The confession, the lawyer has argued, was given by a man incapable of understanding his rights against self-incrimination and only after more than seven hours of interrogation, none of which detectives recorded. He has asked that the confession be declared inadmissible.

Of Hernandez's confession, the lawyer said in court Monday, "It is filled with impossibilities."

For prosecutors, the videotaped confession shown Monday – it had never been seen publicly before – is the testimony of a man who had lived with an ugly, inexplicable secret for much of his life. In court Monday, they highlighted Hernandez's demeanor – calm, forthcoming – as evidence that he was genuinely unburdening himself of a long-hidden guilt. Joan Illuzi-Orbon, a senior prosecutor with the Manhattan District Attorney's office, said that after Hernandez confessed to detectives he disclosed the crime to his wife and daughter.

"They said they'd never seen him so at peace," Illuzi-Orbon said.

The prosecution of Hernandez has re-ignited interest in an infamous case, but also concerns about the phenomenon of false confessions. Prosecutors have offered no additional evidence directly tying Hernandez to Patz's alleged murder, and Hernandez's lawyer, Harvey Fishbein, has highlighted what he argues are reasons to distrust the confession: His client is borderline mentally retarded; he has a history of mental illness; he was held against his will for hours during his initial interrogation; the case had recently been back in the headlines, as police and the FBI in the spring of 2012 undertook a high-profile excavation of a SoHo cellar they thought might hold clues to Patz's fate.

In his opening remarks during Monday's court session, Fishbein said he intended to make clear the confession's details didn't match the accepted facts of the case. Hernandez, he said, is the only person ever interviewed who placed Etan outside the bodega that morning in May 1979. He said Hernandez's claim that he tossed the boy's backpack behind a freezer in the bodega basement isn't credible given that the bodega was searched on at least four occasions in the days after Etan's disappearance. He said the spot on the street identified by Hernandez as the place he put the box with the boy's body was, in fact, a thriving bakery at the time.

"He's wrong all the time," Fishbein said of his client's confession. "He's inconsistent. He's a terrible historian."

Certainly, the taped portion of Hernandez's confession played Monday – an earlier confession made after Hernandez's hours with detectives will be played in the coming days – was rich with curiosities.

Hernandez offers no reason for killing the boy. He recalls no police presence in the neighborhood in the days after Etan's vanishing, and that he simply went to work at the bodega for another year, never questioned by police, never discussing the case with his co-workers, two of whom were close relatives with whom he lived.

He said he remembers the boy wore a dark jacket and perhaps sneakers, but can say little more about his clothes. He describes stuffing the bagged boy into a box no bigger than the length of his own arms and no more than two feet high. He said he "took a walk" the day after killing Etan to check on the box.

"Somebody took the box," Hernandez said on the tape. "I don't know who."

If the case goes to trial, prosecutors are expected to call several people to whom Hernandez over the years allegedly confessed about harming or killing a child. A ProPublica examination of the case against Hernandez, however, found that the testimony of those witnesses is filled with additional confusions or inaccuracies.

And the playing of the confession Monday began to touch upon those confusions. The tape indicated that Hernandez's former wife had told detectives that Hernandez had confessed to her years ago, even before they were married. The prosecutor on the tape tells Hernandez that his ex-wife said he had killed a "muchacho" in a fit of anger after the boy made a pass at him.

On the tape, Hernandez disputes his ex-wife on virtually every point.

The hearing is expected to be lengthy. There will be competing psychiatric testimony. The detectives who interviewed Hernandez may well take the stand.

It is unclear if Stan Patz will be there every day. He has not spoken publicly since Hernandez's arrest. At the start of Monday's hearing, he was joined by his wife. But she left the courtroom when the tape was turned on. Stan Patz stayed, saying nothing to reporters afterward.