A three-month-old baby girl had no obvious injuries when she was rushed from rural Victoria to a hospital in Melbourne, where she later died.
The baby became critically ill on Saturday afternoon while at home with her parents and five-year-old step-sister in Heathcote.
Concerned ambulance officers contacted police when it appeared the girl was unlikely to survive.
She was airlifted to the Royal Children's Hospital but died about 7.30pm on Sunday.
Police have not revealed the cause of her death and post-mortem results are still days away.
The homicide squad's Detective Inspector Mick Hughes said the death was being treated as suspicious and the post-mortem results would shed further light on the case.
"Honestly, when the child was attended to by ambulance there was no obvious sign of injury," Det Insp Hughes told reporters on Thursday.
"She was in critical health, but had no physical injuries."
Insp Hughes described the child's parents as a young, loving couple who had no prior dealings with police or other services.
A man aged in his 20s has been interviewed and released but Insp Hughes would not clarify whether that man is the girl's father.
The baby was the couple's biological child, the older step-daughter coming from an earlier relationship.
Police are unable to say whether reports the three-month-old had been born premature are true, but it will form part of their investigation.
Detectives door-knocked residents living near the child's home on Saturday night, but are appealing for anyone with information about the death to contact police.
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