The mystery surrounding the four-month disappearance of a little boy has taken a dramatic turn with a family member declared a suspect in his likely death.

Gus Lamont was last seen by his grandmother playing on a mound of dirt at a sprawling sheep station in outback South Australia on September 27.

His disappearance from Oak Park Station sparked intensive searches spanning almost 500 sq km by hundreds of police and volunteers using aerial support and mounted units.

Detective Superintendent Darren Fielke on Thursday said initial searches were based on Gus walking off the station and becoming lost.

“This is one of three investigation options that have been explored by the members attached to Taskforce Horizon,” he said, adding the case had been declared a major crime.

“The two other investigation options are focused on Gus being abducted … or whether someone known to him was involved in the disappearance and suspected death.

“We don’t believe now that Gus is alive.”

He said a person who lives at Oak Park Station had withdrawn co-operation in the investigation after “a number of inconsistencies and discrepancies” were identified in the information they provided.

Det Supt Fielke confirmed two grandparents, Gus’ mother and younger brother were on the property when he disappeared.

“I do want to stress, however, that Gus’ parents are not suspects in his disappearance,” he said.

Police drained a dam on the outback property during the search for Gus Lamont. (HANDOUT/SA POLICE)

Investigators searched the homestead on January 14 and 15, seizing items including a vehicle, a motorcycle and electronic devices.

“All of these items are now subject to forensic testing,” Det Supt Fielke said.

After extensive searches, imagery captured by an aircraft and a drone was given to an AI company.

It had produced high-definition pictures which showed every sheep, goat, kangaroo, wedge tailed eagle, wallaroo and animal carcass on the property and one human riding a motorcycle.

“Clearly that’s not Gus,” Det Supt Fielke said.

“What’s the result of all of this? The ground searches and the aerial searches with the images you’ve seen have failed to locate Gus or any items belonging to Gus.”

Detective Superintendent Darren Fielke stressed Gus’ parents are not suspects in his disappearance. (Abe Maddison/AAP PHOTOS)

Family members had been co-operating with police and providing information, which police had reviewed.

“As a result of that, we have identified a number of inconsistencies and discrepancies with that information as it relates to timelines and the version of events provided to us by the family members,” Det Supt Fielke said.

“A person who resides at Oak Park station has withdrawn their support for the police and is no longer co-operating with us.

“I can tell you that Taskforce Horizon members, myself, are very cognisant of how delicate that is and what that means for the family.”

The four-month investigation had “taken many twists and turns” and Gus’ parents were shocked, he said.

“It’s challenging, it’s delicate at times when you’re dealing with family members … our people have been unwavering and kept an open mind,” Det Supt Fiekle said.

Hundreds of police and volunteers using aerial support and mounted units took part in the search. (HANDOUT/SOUTH AUSTRALIA POLICE)

The suspect now had legal representation.

Asked if Gus could have been at the property when police were there, Det Supt Fielke said officers were on scene quickly when they were advised the boy was missing and it was possible.

Police remained focused and determined to locate Gus and return him to his parents, he said.

The initial 10-day air and ground search was one of the largest SA Police had undertaken.

In November, search teams focused on six mine shafts near the property, 40km south of Yunta, using specialist officers, but found no trace of Gus.

A four-day search within a 5.5km radius of the homestead in early-October also ended without any evidence.