For most of the videotaped confession, taken in a small room on the ninth floor of 100 Centre Street, Mr. Hernandez seemed poised and coherent, calmly providing the answers to a crime never solved but never forgotten.

“It was like something took over me,” he said. “Something just took over me and I just choked him.”

Mr. Hernandez, now 53, said the boy collapsed and was unconscious. “He wasn’t dead,” he said. “He was just gasping.” Mr. Hernandez said he stuffed Etan into a garbage bag, “even though he was still alive,” then put him in a cardboard box. He tossed the child’s book bag behind a large walk-in freezer.

Mr. Hernandez hoisted the box on his shoulder and carried it a block and a half away and left it in a doorway in an alley. When he went back the next day, the box had been moved, he said. The body was never recovered; the authorities pronounced Etan dead in 2001.

Mr. Hernandez, dressed in an orange jumpsuit, sat placidly at the defense table as his confession was played on a television set, his face not betraying any concern. In the second row, the missing boy’s father, Stanley Patz, looked severe, worn down and sad, as Mr. Hernandez described suffocating his son. He sat with the boy’s sister, Shira Patz, and other relatives. The boy’s mother, Julie, was in attendance, but left the courtroom before the tape was played.

Mr. Fishbein told Justice Maxwell Wiley that his client was so suggestible because of his intellectual disability and mental problems that he had become convinced — especially following an initial interrogation in Camden that lasted nearly eight hours — that he had killed Etan.

Image Etan Patz’s mother, Julie Patz, leaving court on Monday. Credit... Anthony Lanzilote for The New York Times

“There is no question that when the detectives were finished with my client in the morning of May 23, my client believed what he was saying to the police was true,” Mr. Fishbein said. “The issue is, was it reliable?”