The fire broke out at 1:40 a.m., reaching three alarms and gutting a 3-story apartment at 1066 Decatur Street and a neighboring building in the Bushwick section.
Witnesses said they saw a man running from the scene shortly after the fire started with his skin peeling off.
Officials said the boy who died became separated from his mother while trying to escape the inferno.
The boy was later identified by authorities as 7-year-old Joel Smith.
Seven others, including three children, went to the hospital with injuries.
Investigators believe the fire started on the first floor, where the boy lived, then rapidly spread. The entire building was engulfed in flames by the time firefighters arrived on the scene.
They were still trying to determine what caused the fire.
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