Updated, 8:20 p.m.
WBZ reports police this evening found the body of Kyzr Willis in the water off Carson Beach.
The boy was last seen around 2:05 p.m. near the M Street bathhouse.
State and Boston police launched a massive search that included helicopters and boats and tracking dogs and officers called in from other parts of the city, who looked for him on nearby beaches and as far away as a Dorchester bowling alley.
The Boston Police homicide unit is now investigating his death, as is routine with all unexplained deaths in Boston.
In a statement, Mayor Walsh said:
The City of Boston is devastated to learn of the tragic loss of Kyzr Willis, and I will be working closely with Boston Centers for Youth and Families and the Boston Police Department until we know exactly what happened. My heart is broken for the Willis family and my thoughts and prayers will be with them.