Police are looking for two men that left the scene where a 27-year-old officer died in a road crash involving a suspected stolen vehicle.
The officer was travelling in a police car on routine patrol when the incident happened in Londonderry last night.
Two other officers in the patrol car sustained non-serious injuries.
Police have appealed for witnesses to come forward.
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The pair, believed to be the boy's mother and stepfather, were held after the five-year-old's lifeless body was discovered in a lagoon.