The New York Police Department’s chief of detectives, Rodney Harrison, said on Friday that a Manhattan judge had issued an order to obtain forensic evidence from the 14-year-old who was detained on Thursday. He would not say whether that evidence would include a DNA sample.

Chief Harrison said he expected to get results from the forensic evidence testing in three to seven days. Investigators are monitoring the teenagers, he added.

The police charged a 13-year-old with second-degree felony murder the day after the killing and tried to interview one of the two 14-year-olds. But the 14-year-old — who is not the teenager who the police believe stabbed Ms. Majors — requested a lawyer and declined to give a statement, officials said.

The 13-year-old, whom The New York Times is not naming because he is not being charged as an adult, will be prosecuted in family court on the felony murder charge, meaning he is accused of taking part in the attack and not of stabbing Ms. Majors.

Image Tessa Majors

On Thursday, the police announced a break in the case, saying they had found the second 14-year-old at a family member’s home in the Bronx. But a few hours later, he was released. He had been questioned in the presence of a lawyer, the police said.