Corey Foster was physically restrained at Leake & Watts residential treatment center in Yonkers after a dispute on a basketball court on April 18.
The report said the 16-year-old suffered cardiac arrest "during an excited state while being subdued."
His mother, Sheila, told the Journal News she was disappointed with the finding.
Her attorney said he planned to file papers in court Tuesday seeking the release of a surveillance video that could help identify staffers involved in the incident.
The district attorney's office said the investigation was continuing.
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