Part-time taxi driver Gursewak Dhillon apparently ran out of fuel and was helped by a woman in this Australian city while he was driving around with three-year-old Gurshan Singh Channa in the boot of his car, the police said.
Police intend to speak with the woman they believe stopped to help Dhillon between 12 pm and 3 pm on Thursday, Australian news agency AAP reported.
Dhillon is alleged to have placed the child in the boot of his car unconscious, but still alive.
Gurshan's body was found on Wildwood Road on Thursday night after a massive search following a complaint by his parents who were staying in Lalor, over 20 km away.
Officials believe the woman helped Dhillon who was driving a dark green car somewhere in Melbourne's northern suburbs. She took him to a service station.
Dhillon was a housemate of Gurshan's parents.
It was still unclear as to how the boy was rendered unconscious; an autopsy has failed to determine a cause of death.
Gurshan arrived in Australia in January with his parents. The family was due to return to India as his father Harjit Singh Channa failed to find work in Melbourne.
Singh and his wife Harpreet Singh Channa gave statements to police on Friday but officials say they are not suspects.
Gurshan's death had caused an uproar in India, with New Delhi on Friday describing the incident as "most unfortunate".
There have been a string of attacks on Indians in Australia, with one of them proving fatal. Nitin Garg was fatally stabbed on January 2 while on his way to work in this city.