The boy died in Fremantle Hospital yesterday after the St John Ambulance found him unconscious at a Hilton home shortly before 2pm (WST).
Neither St John Ambulance nor WA police will confirm reports the boy's 24-year-old mother found the pre-schooler inside a washing machine.
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Detective Senior Sergeant Doug Nelson says the boy's mother and several family members have been assisting in their investigation.
"We are now conducting inquiries into what happened at the house and those inquiries are ongoing as we speak," he told AAP.
"I'm not prepared at this stage to comment on what may or may not have occurred inside the house at this stage."
Forensic investigations at the scene and a post-mortem examination have not yet been completed.
The boy's mother and "several members of the family" are being interviewed at Fremantle police station, Sen Sgt Nelson said.
"It's very distressing for everyone who's concerned," he said.
"Obviously, for the family of the young boy who's passed away, the first responders who have to attend, the St John staff, the medical staff and also the young police officers and the detectives who have to deal with it.
"It's distressing for everyone. So, obviously, we have things in place to assist people who are affected by it where we can."
