Police said the boy, named Ronan, was unconscious and in a critical condition when he was taken to the Women’s and Children’s Hospital at about 6pm on June 7.
Ronan had suffered head and other injuries and died two days later.
Detective Inspector Mark McEachern said Ronan’s injuries are believed to have been caused by an assault.
“In the two days before Ronan was taken to hospital he was being cared for by friends who are known to his parents, at an address in Ferguson Street, Glenelg North,” McEachern said.
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“When he was returned to his father’s house at Ottoway at 4pm on the Wednesday he was unable to be roused and an ambulance was called.”
Police are asking if anyone heard or saw anything involving a young child in Ferguson Street on the afternoon of June 7.
This includes anyone that may have dash cam footage in Ferguson Street that afternoon.