At Friday’s hearing, Detective Vincent Signoretti testified that the 13-year-old boy, whom The New York Times is not naming because he is not being charged as an adult, told the police that he and two other teenagers had gone to Morningside Park specifically to rob people.

“They followed a man with the intention of robbing him and decided not to,” Detective Signoretti said.

The trio later spotted Ms. Majors in the park, he said. The boy told Detective Signoretti that he watched his two friends grab the student, put her into a chokehold and remove items from her pockets, the officer testified.

Then, shortly before 7 p.m., the boy watched as his friend slashed the young woman with a knife and feathers from the stuffing of her coat came flying out, the detective testified.

The boy was arrested on trespassing charges on Thursday evening in a building near the park and interviewed by detectives with his uncle present, officials said. His statements led investigators to the other suspects, one law enforcement official said.

One of those suspects, who is 14, was detained and interviewed on Friday, the official said. A second official said that he had a lawyer present when he was interviewed.

The third suspect is believed to be the person who stabbed Ms. Majors and as of Friday evening was still being sought, the first official said.