Someone who lived with four-year-old Gus Lamont is now considered a suspect in his disappearance, SA police said on Thursday as they declared the case a major crime.

Four-year-old Gus Lamont disappeared from his family’s outback home in South Australia more than four months ago, sparking what SA police called “one of the largest, most intensive and most protracted searches” they had ever undertaken.

Mounted police, police divers, defence and emergency services personnel, drones and trackers searched the 60,000 hectare Oak Park station, near Yunta, which is about 300km inland from Adelaide.

Detective superintendent Darren Fielke, the officer in charge of major crime, said police had effectively ruled out Gus wandering off and Gus being abducted. He said they were now investigating someone in his household – but stressed the suspect was not one of his parents.

Police had “identified a number of inconsistencies and discrepancies” in the information from the family members, Fielke said.

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“As a result of these inconsistencies, and investigations into them, a person who resides at Oak Park station has withdrawn their support for the police and is no longer cooperating with us,” he said.

“The person who has withdrawn their co-operation is now considered a suspect in the disappearance of Gus. I do want to stress, however, that Gus’s parents are not suspects in his disappearance.

“You appreciate at this time I cannot make any further comment about the suspect, given that this is now a criminal investigation and a declared major crime. What I can say, however, is that we’ll continue to thoroughly and meticulously investigate the disappearance of Gus until we get an outcome.”

Gus, who a family friend described as shy but adventurous, was playing outside the family sheep station at 5pm on Saturday 27 September.

The blond, curly haired little boy was wearing a blue T-shirt with a yellow Minion on the front, a grey sun hat, light-grey long pants and boots.

When his grandmother went to call him in half an hour later, he was gone. The police searches waxed and waned as different pieces of information came to light. They drained a large dam, investigated abandoned mine shafts and heard evidence from survival specialists about how far he might have roamed.

They also battled AI-generated misinformation and people calling into a hotline with “opinions”.

A tiny footprint found in the days immediately after his disappearance yielded no useful information.

“The community should take comfort that no stone is being left unturned in this investigation,” Fielke said.