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Nevayah still remembers the feel of the handcuffs. They were foreign to her; she had never been in trouble.

A latecomer to New York City’s foster care system, Nevayah had been signed over to the Administration for Children’s Services when she was 16. Rather than enter a group home, she told her caseworker she would prefer to live with her mother in Ohio. Eager to start school, she bought a bus ticket, made it to Cleveland and phoned the agency to let them know that she was safe.

Per protocol, A.C.S. said, they would send the authorities to make sure the home was suitable.

But when the police arrived that night, they told Nevayah that New York Family Court had issued a warrant for her arrest. Graciously, she said, officers waited to handcuff her until she was in the back of their patrol car. By midnight, she was in a cell.

In Family Court hearings every month, the A.C.S. is quietly being granted arrest warrants to detain foster children like Nevayah, whose only transgression is leaving the agency’s care. The unusually draconian strategy has little precedent in any state’s foster care system, and it is unclear if the A.C.S. even has the authority to use such warrants under New York State law.