A woman and a child died after a car was found sunk in a pond near an airbase.
Police rushed to the pond near RAF Wittering in Cambridgeshire at 6.20pm on Saturday following reports that a blue Ford Focus was in the water.
The boy was taken to Peterborough City Hospital for treatment, but later died, while the woman was confirmed to have died at the scene.
Police say they do not know at this stage how long the car had been in the water before they were called.
The age of the child has not been revealed while formal identification procedures are carried out.
A police spokesperson said: "At about 6.20pm police were called with reports the blue Ford Focus had been spotted in the water next to Old Oundle Road, to the rear of RAF Wittering.
"Police, fire crews and paramedics attended the scene and the woman and child were removed from the vehicle.
"The boy was taken to hospital for treatment, but later died, while the woman died at the scene."
Anyone with information can call police on 101 quoting 414 of 3 September.