IS this the breakthrough in the missing Beaumont children mystery police have been waiting decades for?

Major crime detectives in South Australia have revealed they are looking into a new lead, and have even been given the name of a suspect.

The development comes as police release the official file to the public in a bid to inject life into one of the most baffling missing persons cases in Australian history.

Next week, on Australia Day, marks the 50th anniversary since Jane, 9, Arnna, 7, and four-year-old Grant Beaumont went missing in 1966 after failing to return from a trip to Glenelg beach.

Major Crimes Detective Superintendent Des Bray — who most recently led the investigation into the death of two-year-old ‘suitcase’ murder victim Khandalyce Pearce-Stevenson — said police had never closed the Beaumont case and were currently looking into a new lead involving a now-dead suspect.

But he warned there was a good chance it could come to nothing, like hundreds of other tip-offs about the children over the years.

“We had a phone call to the office from somebody who had been told something by somebody about who committed this offence,” he told ABC radio on Tuesday.

“They firmly believed that the person they’re nominating has done it, so we’ve sent investigators to investigate those claims. That will take a few days to do.

“Whether it comes of anything or whether it’s like the hundreds of people who’ve been put forward before and it amounts to nothing remains to be seen.”

Det Supt Bray said it was a “credible” tip.

“It’s a person who’s died and that makes some of our inquiries hard when people are nominated who are already deceased because obviously they’re not there to interview,” he told the Nine Network.

The newly-released file contains the initial missing persons report and includes never seen before images of the children from shortly before they disappeared.

The images show the trio laughing and playing while the first police notes record that searches took place on boats and nearby properties, as well as the anxiety experienced by the children’s parents, Channel Seven reported.

Police have received 159 CrimeStoppers calls about the children — one every four days — over the past two years.

But time may be running out. While dozens of people are still considered persons of interest in the case, the passage of time means the window of investigation is closing, as witnesses die and lose their memories, Mr Bray said.

Police believe the Beaumont children case may be linked to the disappearance of Joanne Ratcliffe, 11, and four-year-old Kirste Gordon, who went missing from Adelaide Oval in 1973.

The Beaumont case forever changed the way Australian parents supervised their children.