"Although police have no information that suggests there were other passengers in the vehicle, we will continue a precautionary land and air search until we are satisfied that all areas of interest have been sufficiently canvassed", Southern Desert Division Superintendent Jody Nobbs said.
Police located the bodies of a two adults and a child on a road some 300km northwest of Alice Springs on Wednesday.
Earlier, police confirmed a man and woman, both aged 19, and a three-year-old boy were found dead about 4.5 kilometres from their broken down vehicle.
"At this point, we hold grave concerns for the welfare of this child", superintendent Jody Nobbs told reporters Thursday. They said the three people may have died from exposure a couple of days ago and not as a result of the crash.More news: Porsche 911 Is Completing Final R&D And Testing Phases
NT Police were alerted to the deaths, which were not believed to be suspicious, when a person walked into a local health clinic and raised the alarm just after midday yesterday.
Their vehicle was about 70km from Willowra and was last seen leaving the township on Friday bound for Yarra Yarra.
Nobbs said police still couldn't rule out the possibility that there had been other passengers in the auto when it broke down.
Remnants of food and water were found in the vehicle.More news: I'm not that stupid - Charles won't 'meddle' when he is king
First reports to police on Wednesday indicated the vehicle had crashed but once investigators arrived it was clear there had not been a crash.
Family members have not yet authorised the released of the child's name, but told police community members in Ali Curung, Ti Tree and Willowra, who "are aware of his particulars".
This morning, police said a 12-year-old boy, who they believed was with the group, was not found among the dead.
Police are calling on anyone with information in relation to the whereabouts of the child to contact them immediately on 131 444 or Crime Stoppers anonymously on 1800 333 000.More news: Nintendo to remove offensive Native American designs from Super Smash Bros. Ultimate
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