A 17-month-old Adelaide boy died of head injuries sustained during a suspected assault earlier this month, with police declaring it a major crime and launching a murder investigation.

No one has been charged over the death.

Detectives are focusing on a two-day period when the boy was being cared for by family friends.

Identified as Ronan, the toddler was returned home on June 7 and soon after taken to the Women’s and Children’s Hospital when his father could not rouse him.

He died on June 9.

Detective Inspector Mark McEachern says the boy’s injuries would not have been obvious when he was returned to his parents.

“Ronan’s been returned home, apparently asleep. We suspect he sustained injuries sometime prior to that,” Inspector McEachern said on Tuesday.

“We have been provided with an explanation of how Ronan’s injuries occurred.

“We’re not satisfied with the explanation that’s been given to us.”

– AAP