Cold case: Four decades on, what happened to baby Jason?

Darren Shannon was eleven months old when he was last seen on Saturday 9 June 1973 at his grandparents house. Photo / Supplied

If he's still alive, Darren Jason Shannon is 44 years old now. And his mother could have stopped wondering.

Instead, more than four decades on, Michelle Swift, 67, clings to a memory and minuscule hope the baby snatched form her is still alive, and prays for a breakthrough.

And that's just what South Australian police are hoping for as they renew their efforts to solve one of their most puzzling cold cases: the disappearance of the 11-month-old boy whose mother knew him as Jason.

It's 43 years since Jason's father, John 'Barry' Shannon snatched his son from his grandparents' house in the northern Adelaide suburb of Elizabeth West during an access visit on June 9, 1973.

Just hours later, Mr Shannon, who had a history of mental health problems, died in head-on collision car crash just north of Adelaide.

But there was no sign of baby Jason in the wreck. Or the surrounds. No sign of him anywhere. Not a trace.

The case made headlines across Australia. But investigators made little headway.

To this day it is not known what happened to baby Jason, and the case is now in the hands of the SA Police Major Crime Investigation Section.

"Police conducted an extensive search at the time - including checking hotels, motels and petrol stations," said Detective Sergeant Cameron Georg, from Major Crime, as police renewed their plea for public help recently.

"Plus the case received significant media attention, but the child was never located.

"He would be 44 now and his mother, Michelle, has never stopped wondering what happened to her son.

The case is one of a swag of mysteries being investigated by SA police via long-term cold case campaign Operation Persist.

In Baby Jason's case, police continue to work on two theories: first, that he was killed by his father shortly after being snatched, and his tiny body disposed of ahead of the crash which killed his father.

The second is that Baby Jason may have been given to someone else to care for and raise.

"At the time police held fears he could be dead - murdered that night and his body disposed of, but it was hoped that he had been left with friends." said Det Sgt Georg.

"John Shannon's parents and brother are deceased now, but if a friend of the family knows of a 'secret' baby, or helped raise this child, we would appeal for them to come forward to lay this mystery to rest."

Michelle Swift is now 67, and told the Adelaide Advertiser recently she still vividly remembers the night her son disappeared.

"I can remember Barry came to the house twice that day, which was unusual," she said. "He came early afternoon and again in the evening."

She said her own father, tried, but failed to catch her estranged husband as he fled with Baby Jason, something which "played on Dad's memory for a long time".

Michelle and her parents went back to the UK in 1974.

She has remarried and has a son, but admits she cannot extinguish the faint hope she might find out what happened to Jason and that he may still be alive.

"I think any human being would have that hope," she said.