Mr. Moeller asked the jury to pay close attention to Mr. Lewis’s behavior in the video of his confession, noting that he, though an adult, had asked to speak to his mother and to watch cartoons while in police custody.

Mr. Lewis lived with his mother and had graduated from a school for students with learning disabilities. He appeared confused at times during the videotaped interview, and mumbled through some of his responses.

One juror, who requested not to be named for fear of harassment, said there were some undecided jurors when deliberations started. He and two other jurors requested to review the videotape of the confession.

“You always see cases where the wrong person is put behind bars and then 30 years later their whole life is gone and you find out they’re not guilty,” the juror said. “That was in the back of my mind.”

In the end, however, he said he was persuaded by the other jurors that the confession was genuine and that the scientific evidence backed it up.

Shortly before the verdict on Monday, the case took an unexpected turn when defense lawyers received an anonymous letter from a person who claimed to be a police officer. That person said that before the police turned their focus toward Mr. Lewis, they had collected DNA samples from at least 360 black men who had previously been in custody in parts of Queens and Brooklyn. The letter also said the police initially thought two white men had killed Ms. Vetrano.

Mr. Lewis’s lawyers asked Justice Michael B. Aloise for a hearing to determine if prosecutors had withheld evidence favorable to Mr. Lewis, and for a mistrial. Mr. Leventhal, the prosecutor, argued that the letter was sent by someone “trying to derail this case on the eve of summations.” Justice Aloise denied both of the defense’s requests.