UPDATE: The boy was found dead on Monday on the shore of Spectacle Island, police said.
State police say they are searching for a missing child on Castle Island and the surrounding ocean.
Police first received the call for help around 8 p.m. on Sunday.
Crews are specifically searching by Fort Independence along the harbor walk, police said.
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Boston police, Boston fire, and Boston emergency medical staff are assisting in the search for the missing child, who is a 4-year-old boy from South Boston, according to officials.
Multiple people who were at the park at the time of the call say personnel arrived quickly, and first started searching by playgrounds before spreading out the search to the water.
Authorities say they are utilizing patrols, the Mass. State Police Marine Unit, Dive Team and Air Wing to search for the missing child.
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This is a developing story.