The grandparents of Gus Lamont say they are “absolutely devastated”, after SA Police declared the disappearance of the four-year-old boy a major crime.

On Thursday, police said they had a suspect who lived at Oak Park Station, where Gus was last seen on 27 September, stressing that suspect was not one of the boy’s parents.

In a statement, grandparents Josie and Shannon Murray said the family had “co-operated fully with the investigation and want nothing more than to find Gus, and re-unite him with his mum and dad”.

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

The lawyers representing the pair said they would not participate in any interviews or comment further.

At an update on the case on Thursday, Major Crime officer in charge Detective Superintendent Darren Fielke said police executed a warrant at the property in January, conducted a forensic search and seized items.

“A person who resides at Oak Park Station has withdrawn their support for the police and is no longer co-operating with us,” he said. “The person who has withdrawn their co-operation is now considered a suspect in the disappearance of Gus.”

He said police began to focus on the suspect, after they identified “a number of inconsistencies and discrepancies” with information “as it relates to timelines and the version of events provided to us by the family members”.

“We have a suspect – who we believe is known to Gus,” he said.

“[SA Police’s] Task Force Horizon members, myself, are very cognisant of how delicate that is and what that means for the family.”

Turn in investigation ‘not surprising’

Criminologist Vincent Hurley – who was a New South Wales police officer for nearly 30 years – said, while the events of the past 24 hours may surprise the public, it would not be for those in the law-enforcement space.

“As a former police officer for nearly 30 years, the police would have in the back of their mind a number of different alternatives… so it’s not that surprising to police or former police, but it would be surprising to the members of the public, because it’s literally come out of the blue,” he said.

Hurley said there were a number of possible reasons why the police released the latest information on Thursday, but said “everything the police do when they front the media is really carefully thought out and planned”.

“They don’t front the media just on a whim or just for the sake of it… so these things, whilst they might seem on the surface very routine, they’re not.

“They are done at a specific purpose. They are done at a specific point in time for a specific purpose, and that purpose and that timing is unknown to us, because we only see what happens at the front in the media.

“We don’t know what’s going on in the command post or in the operational police centre, for example, so we are largely hidden from what’s going on with the police and the way they’re going about the investigation.

“That can add a degree of frustration to the public, because they think the police aren’t actually doing something or they’re not making progress.”

Hurley said he understood why some members of the public may be critical of police’s initial focus on the theory that Gus had wandered off, but said police “never follow one particular path of investigation”.

“They would have, from the outset, looked at a number of different possibilities,” he said.

Police wanted to show they ‘exhausted all leads’

Central Queensland University criminologist Xanthe Mallett said information provided by police at Thursday’s media conference was “very detailed”.

“I think they really wanted the public to know that they’ve really exhausted all of those investigative leads,” she said. “It’s been very quiet over the last few weeks and months, and I think people were asking questions as to what was going on inside that investigation.”

Dr Mallett said, while police were possibly a “very long way” from laying charges, they had presented a “clear snapshot” of the case, as it stood.

“There are only a very small number of options as to what happened to Gus anyway and I think they have been very clear in how they’ve reached the conclusions they have,” she said.

“They’ve employed some really quite innovative technology in the search for Gus, in terms of the drone searches, the AI that South Australia Police have not used before, to really determine who was on that homestead at the time.”

Community coming to terms with news

Locals at Peterborough, a township located 80km from Yunta, where the family’s Oak Park station, told the ABC they were not surprised when police announced Gus had likely simply wandered off.

Leonie Polomka said, although she was shocked by Gus’s disappearance, she always believed there was more to it.

“[We were] very shocked when we heard about it, yeah, it was just too close to home,” she said.

“My heart goes out to [the family]. I just couldn’t imagine what they’re going through.

“It’d just be so hard, yeah, not knowing and not having your little boy there.”

Another local, Herbie Fielding, said he hoped Gus could be found and laid to rest.

“That poor little boy was so sweet and innocent, and [it’s] very tragic,” he said.

Bernard Misztal, a visitor from Queensland, said someone likely knew something about what happened.