The announcement was widely expected for several days, but it still devastated the family of Avonte Oquendo, who was 14 and had a form of autism that made it impossible for him to speak.
Avonte's mother, Vanessa Fontaine, was inconsolable, said her lawyer, David Perecman.
"Now that the inevitable, unfortunately, has occurred, undoubtedly she'll go through a metamorphosis of a sort, and I'm sure she'll get good and angry," he said. Perecman said the family intends to file a wrongful death lawsuit against the city, alleging that school officials failed to monitor the boy or call the police quickly enough when he left the school.
"There were so many things that went wrong, it befuddles the mind," he said.
The discovery of Avonte's partly decomposed body on the riverbank, about 11 miles from where he vanished, was a sad end to a massive citywide search that included hundreds of officers, marine units and volunteers. Missing person posters were plastered on lampposts and placed on car windshields throughout the city. Announcements were made for weeks on city subways, imploring people to contact the police if they had any information.
The medical examiner's office used DNA tests to identify the remains discovered Thursday. Further study was needed to determine the cause and manner of death, the office said.
Avonte had been missing since Oct. 4, when he walked out of his school toward a park overlooking the river.
One investigative theory was that Avonte might have tumbled into the river near the park, though his family has said he was fearful of water. It wasn't clear how his remains had traveled so far, but authorities noted that the East River is a tidal strait with strong currents that reverse flow many times a day.
The family first said in October that it would sue the city. At the time, city authorities had defended the school safety officer who last saw the boy, saying she told him to go back to his classroom and he left the hallway.
Carmen Farina, the city's newly appointed education chancellor, said she was heartbroken.
"As chancellor, I am determined that we learn every lesson we can from this terrible tragedy and do everything in our power to prevent incidents like this from ever occurring again," she said.
The city's law department called the boy's death a tragedy and said its attorneys would review the lawsuit once it's filed.