Asked what about Mr. Hernandez’s confession had led detectives to find him credible, Mr. Kelly responded, “The fact that he had told this story to others in the past, and the specificity of what he said in the confession.” He said he did not know what the motive might have been.

Mr. Hernandez, 51, was charged with second-degree murder by the police. Mr. Kelly said he expected the Manhattan district attorney, Cyrus R. Vance Jr., to present Mr. Hernandez for arraignment on Friday, but could not say what charges would be filed.

Under the law, prosecutors will have to bring Mr. Hernandez before a grand jury within six days of the arrest and present sufficient evidence to convince them to vote for an indictment, or hold a preliminary hearing, an extremely rare occurrence.

And it was unclear on Thursday what evidence, beyond Mr. Hernandez’s confession, the prosecutors have in hand. Mr. Kelly acknowledged that there was no physical evidence implicating Mr. Hernandez, though he said the investigation was continuing.

Mr. Hernandez, who was 18 at the time Etan vanished, worked as a stockboy in a bodega at 448 West Broadway that is now an eyeglass store, Mr. Kelly said. Etan disappeared on the first morning his parents allowed him to walk alone from the family’s home on Prince Street to a school bus stop on West Broadway.

Mr. Hernandez was working in the basement, which had a separate door to the street, Mr. Kelly said. Etan was at the bus stop when Mr. Hernandez led him away and to the basement, Mr. Kelly said.