Two members of Gus Lamont’s family are “not cooperating” with the investigation into the missing boy, South Australia’s most senior police officer says.

There has been no trace of four-year-old Gus since he vanished from his family homestead in SA’s remote north in September last year.

A month ago, SA Police declared his disappearance a major crime, saying he was likely dead. They said a person who lived at the Oak Park station was no longer co-operating with police and had withdrawn their support for the probe.

That person was considered a suspect, police said.

The next day, Gus’s grandparents, Josie and Shannon Murray, issued a statement saying the family had “cooperated fully with the investigation”.

On Wednesday, SA Police Commissioner Grant Stevens said “members” of the family were “not co-operating”.

“As far as I’m aware, it’s status quo, from the most recent reports. We are still working with Gus’s mum and dad and there are other members of the family who are no longer co-operating,” he told ABC radio.

Stevens said investigating the little boy’s disappearance – already the biggest search police in the state – remained a “top priority”. Officers from the special taskforce in charge of the probe would return to Oak Park at a later date.

“The taskforce, which comprises over a dozen people, detectives, forensic specialists, will clearly be looking at any opportunity to provide closure to Gus’s mum and dad,” Stevens said.

“There has not been one single piece of evidence that we have located during the searching exercise, the most extensive search I think arguably in the history of South Australia, that has given us any indication that he did wander off.”

The ABC reports that, following Stevens’ comments, an SA Police spokesperson said “that two family members are only communicating [with police] via their legal representatives”.

Asked if there were any further suspects, the spokesperson said they were “unable to make any further comment”.

In February, following a search at Oak Park, more than 300 kilometres north of Adelaide, Josie Murray was charged with unrelated firearms offences. A court appearance is set down for May 6.

Extensive searches for the little boy have included draining a dam. Photo: SA Police

Last week, Gus’s heartbroken parents released a statement through SA Police pleading for anyone who knows anything about his disappearance to come forward.

“We are united in our grief, and we are united in our search for answers about what happened to our little boy, Gus, who means everything to us. Our lives have been shattered, and every moment without him is unbearable,” Josh and Jess said.

“We know someone out there may have information. If someone knows what happened, we are pleading with that person – or anyone who may have seen or heard anything – to please come forward. Even the smallest detail could give us the answers we so desperately need.”

SA Police have returned to the remote Oak Park Station numerous times since September, searching buildings and structures and other locations adjoining the property.

They have searched old mine shafts and inspected an outhouse where fresh cement had been laid, as well as a water tank at a neighbouring property.

A police helicopter assisted along with a cadaver dog but no evidence has been located.

Previously dams have been drained and investigators have seized items from the station homestead, including a vehicle, motorcycle and electronic devices.

The intensive searches for Gus have spanned almost 500 square kilometres and involved hundreds of police and volunteers as well as aerial support and mounted units.

The search for Gus has covered nearly 500 square kilometres. Photo: SA Police

On Wednesday, Stevens said the taskforce would continue “whilst there’s work to be done”.

“Gus Lamont is one of the top priorities for South Australia Police,” he said.

“The resources that are necessary will be kept on the taskforce until such time as the assessments are that would be a waste of those resources because of the diminishing work that can be undertaken by a team of that size.”

He said police would definitely return to Oak Park.

“I can’t say when, I can’t say what the reasons will be, that’s entirely up to the task force. But the work is ongoing,” he said.

Stevens said Gus’s parents were enduring a horrendous time.

“You can’t imagine what this family is going through. As they say, this must be their darkest days,” he said.