A firearm was recovered at the scene where a retired police inspector and his female partner were found dead on Saturday morning in Wiltshire, police have said.
Detective Chief Inspector Ian Saunders from Wiltshire police has said: "The investigation continues to try and establish what led to this tragic incident.
"We have recovered a weapon from the scene. My working assumption is that weapons were discharged and the victims had sustained some form of gunshot wounds."
Post-mortem examinations will be carried out today.
Officers are not looking for anybody else in connection with the shootings, which happened on Moonrakers estate in Devizes.
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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.