Investigators also have a grainy video that shows a scuffle at the bottom of the steps where Ms. Majors was attacked and a person “making poking motions toward the victim,” Detective Wilfredo Acevedo testified at the probable cause hearing on Tuesday.

The video came from a camera on top of a security booth on Morningside Drive, he said. It provides a view of the robbery from a southeast angle and does not show the 13-year-old touching Ms. Majors or taking anything from her.

Detective Acevedo said that the 13-year-old had told him that he and two 14-year-old friends had gone to Morningside Park after dark to mug people when they spotted Ms. Majors walking alone along the steps shortly before 7 p.m. The park, he said, was “extremely dark.”

One of the two 14-year-olds grabbed Ms. Majors from behind, the detective said. Then they rifled through her pockets and took a small plastic bag away from her. Ms. Majors fought back and yelled for help. One of the boys, who had a knife, stabbed her in the torso and slashed her on the face, Detective Acevedo said. She bit one boy’s finger, another officer said last week.

The 13-year-old described to the police how the feathers from her coat fluttered to the ground. The boys fled. She staggered up a flight of stairs and out of the park, where a police officer found her, bleeding and gasping on the sidewalk.

Questioning Detective Acevedo, Ms. Kaplan, the public defender, brought out that the 13-year-old had repeatedly said during the interview that he did not know his friends were going to rob Ms. Majors. “He told you it about 10 times, is that correct?” she asked.

“That is correct,” the detective said.

Video taken from cameras at 116th and 118th Streets and Morningside Avenue shows the boys trailing a man shortly before they encountered Ms. Majors. The 13-year-old told detectives that the group had decided against robbing the man, the police have said.