Gus Lamont’s parents had split up just months before the toddler vanished from his grandparents’ outback sheep station, a family friend has revealed.

On Thursday South Australia Police said they believed the four-year-old was dead, declared his disappearance a major crime, and revealed they had identified a prime suspect within his family.

Detective Superintendent Darren Fielke said detectives suspected that a family member living at Oak Park Station – a 60,000ha South Australian property about 40km south of Yunta – was involved in Gus’s disappearance on September 27.

However they stressed that Gus’s mother and father, Jessica Murray and Joshua Lamont, were not suspects.

It can now be revealed the couple had separated before Gus vanished, with Ms Murray and her newborn son, Ronnie, living with her parents, Shannon and Josie Murray, on the remote station.

Mr Lamont had moved into a ramshackle farmhouse which he was renovating in Belalie North, near Jamestown, 130km west of the Murrays’ station.

A family friend told the Daily Mail their relationship broke down shortly after Ronnie’s birth, but before Gus disappeared.

‘I haven’t seen Josh since it (Gus’s disappearance) happened, and I don’t know where he is now,’ said the local. ‘But (Josh and Jessica) weren’t together when it happened.’

Gus Lamont’s parents, Jessica and Joshua, pictured in 2019

Gus’s September 27 disappearance has now been declared a major crime

The friend revealed the couple had never married, as new details of Gus’s mother’s own childhood were also revealed for the first time.

Several sources have told the Daily Mail that Mr Lamont attended Oak Park station two days after his son vanished.

Gus’s devastated mother, Jessica, 39, has never spoken or appeared publicly since her son vanished, but locals say she is ‘shocked’ by the police statement last week.

Locals say she is shy, quietly spoken, and was rarely seen in the area even before Gus went missing.

Ms Murray grew up in Jamestown, population 1400, where the Murrays once owned a home, two hours’ drive from the sheep station her parents later inherited from Ms Murray’s grandparents Vincent and Clair Pfeiffer.

Publicly available yearbooks at the town’s library showed Jessica attended Jamestown Community School until at least 2002, when she was in year 10.

The source added that Jessica’s biological father Josie – formerly known as Robert and nicknamed ‘Snow’ – began her gender transition while they were living in Jamestown.

Joshua has been living in Adelaide since his son vanished; seen in November

Gus’s grandparent Josie Murray pictured in Jamestown in November