The grandparents of Gus Lamont have issued a statement a day after police confirmed they have a suspect and declared the disappearance of the four‑year‑old a major crime.

Police said the case is formally a major crime more than four months after Gus vanished from a remote Yunta sheep station on September 27.

In a joint comment shared by their lawyers, Josie Murray and Shannon Murray said they were “devastated” by the announcement.

“We are absolutely devastated by the media release of SAPOL Major Crime,” they said.

“The family has cooperated fully with the investigation and want nothing more than to find Gus and reunite him with his mum and dad.”

The comments come after police said the person of interest is not either of Gus’s parents, however, “inconsistencies and discrepancies” were found in the information provided by other family members.

“As a result, a person who resides at Oak Park Station has withdrawn their support for police and is no longer cooperating,” Detective Superintendent Darren Fielke said on Thursday.

“That person is now considered a suspect in the disappearance of Gus.

“A comprehensive forensic search was conducted of the Oak Park Station homestead, and a number of items were seized, including a vehicle, a motorcycle and electronic devices.”

The grandparents have both hired separate lawyers, with the legal firms saying “Andrew Ey and Casey Isaacs can confirm that we act for Josie Murray and Shannon Murray respectively”.

They also confirmed their clients would not be be participating in any interviews or commenting any further on the investigation except for the brief joint comment.

Gus was last seen playing outside the Oak Park homestead while his grandmother cared for his younger brother inside, according to his family.

At 5.30pm when his grandmother went outside to call him in, Gus was gone.

Extensive searches over several months involving police, SES volunteers, army personnel, drones and helicopters and Indigenous trackers have proven fruitless during eight separate large scale searches.

A single footprint discovered about 500m from the property is the only physical clue believed to have been found of Gus.

Police said the major crime declaration reflects the seriousness of the investigation, and that they were confident Gus didn’t walk off on his own.

A person with close ties to the household of missing four-year-old boy Gus Lamont has been identified as a suspect in his disappearance, with police declaring the case a major crime.

Gus was reported as last seen playing on a mound of dirt about 5pm on September 27 at his family home, the Oak Park Station homestead, which is about 43 kilometres south of Yunta in South Australia.

Police yesterday confirmed a person who has close ties to the household, who is not one of his parents, was speaking with detectives but then “withdrew co-operation” and is now considered a suspect.

Gus was last seen playing on a mound of dirt about 5pm on September 27 last year. (South Australia Police)

No arrests have been made.

Detective Superintendent Darren Fielke said investigators had identified “inconsistences” regarding the timeline of Gus’ last movements.

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“As a result of these inconsistencies, and investigations into them, a person… 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.”

Fielke could not provide further information on the suspect.

The “discrepancies” regarding information gathered were identified in mid-January.

“Only as recently as 14 and 15 of January when we attended [the homestead], did we get more information that we have analysed and reviewed, which again has highlighted some discrepancies,” Fielke added.

A search warrant was executed at the Oak Park Station property at this time and a number of items were seized, including a car, a motorbike and electronic devices.

“I anticipate there will be further searches at Oak Park Station and at several sites on an adjacent national park to locate Gus as new information and intelligence comes to hand as the investigation continues,” Fielke said.

Only three people were on the property at the time of Gus’ disappearance, Fielke said.

He did not rule out the possibility of an arrest being made.

He revealed the investigation had centred on three theories – whether Gus was abducted, whether someone he knows was involved in his disappearance or suspected death and whether he did wander off from Oak Park Station.

Fielke said detectives from Taskforce Horizon have a “high level of confidence” he did not wander off.

Investigators have also found no evidence to suggest Gus was abducted.

He also confirmed Gus’ disappearance is now a major crime when providing an update to the media.

A full-scale ground and air search of the property and surrounding area was launched after Gus went missing. (9News)

The search for Gus

No evidence of Gus has been uncovered despite extensive searches of the property, which Fielke said has been the “largest and most intensive” missing person’s search ever undertaken by SA Police.

A full-scale ground and air search of the property and surrounding area was launched after Gus went missing.

Officers returned to the area surrounding Oak Park Station to search for Gus’ remains on February 2 but uncovered no evidence.

“The foot search has been unprecedented and the resources that have been used have been significant,” Fielke said of the search efforts.

“We’ve had deployments to Oak Park Station for over 20 days as part of that operation and there’s been at least eight separate searches conducted.”

Fielke said the owners of Oak Park Station and adjoining properties had been conducting their own searches.

Intensive searches have so far failed to locate Gus and the latest was suspended last year. (9News)

The search area has totalled 706 square kilometres, with the assistance of 80 Australian Defence Force soldiers, drones, mounted officers and aircraft.

Around 163 SA Police officers have been deployed for the search.

A three-day search of six mine shafts near Oak Park Station also failed to uncover any evidence.

Police received more than 150 Crime Stoppers tips about the missing boy.

Detectives returned to the remote property last month to speak to the family.

“We’ve taken statements with the family, we’ve had to go through those statements with the family, and that’s just part of the normal investigational process that we go through,” Fielke said in January.

“We continue to engage with them.

“They went through a pretty horrid Christmas without Gus, and we check-in with the family regularly.”

Police previously stressed that no part of the investigation had turned up any suggestion of foul play.

Detective Superintendent Darren Fielke. (9News)

Lamont was reported as last seen by his grandmother at the family’s Oak Park Station homestead.

About 30 minutes later, he had rerportedly vanished and a full-scale search was launched.

The four-year-old with blond curly hair was described as a shy but adventurous child.

His family have said in a statement they are “devastated” and “deeply distressed” by his disappearance.