The South Australian detective who collared outback killer Bradley Murdoch has spoken for the first time.

Geoff Carson told 9News how he linked a mystery man in the Riverlands to the monster who killed Peter Falconio and shares his theory on what Murdoch did with the backpacker’s body.

Now retired, Carson was the small-town cop who solved a national mystery.

“At the start of the new century, it was the most talked about crime at the time, who would’ve done such a brazen act,” he said.

The breakthrough began with a single phone call.

“He says, ‘I don’t care what you’re doing, I need to see you right now’, and he said the name of his daughter and he said that she’d been raped.”

In South Australia’s Riverland, a woman and her 12-year-old daughter had been abducted by a violent monster.

They were bound and gagged, and put in the back of a truck that belonged to Murdoch.

After a 20-hour ordeal, the victims were paid for their silence and let go.

When Carson took their statement, something clicked.

“Little pocket notebook, I wrote down, halfway down the page, Falconio, then a question mark, it just rang to me straight way,” Carson said.

The previous year British backpackers Falconio and Joanne Lees had been ambushed along a remote stretch of highway in the Northern Territory.

Lees escaped into the scrub after being bound with the same cable ties as the Riverland victims.

“In 40 years of policing I hadn’t come across someone who’d done that before, so it was a unique type of behaviour, I thought,” Carson said.

When Murdoch was arrested over the abduction of the mother and daughter, Carson made contact with the Northern Territory.

“I said I reckon I got your fella,” he said.

But NT Police had already crossed Murdoch off their list.

So Carson took matters into his own hands.

He bundled up cigarette butts and milk cartons marked with the killer’s DNA, evidence that would lead to a match and a warrant for Murdoch’s arrest.

For the Riverland rapes a jury found there wasn’t enough evidence to convict Murdoch.

But on the day he was acquitted police were waiting at Adelaide’s Supreme Court.

The monster was charged for an outback murder.

But Carson remains determined to solve that mystery once and for all.

He says he was given one clue, a remark Murdoch had made to a friend.

“He said to this old fella, ‘If I was ever going to get rid of a body, I’d put it into the grate off on the side of the road, like the flood mitigation, bury it in one of those because no one would ever find it, no one would ever dig in there’,” Carson said.

Falconio’s remains may still be missing but the case isn’t closed.  

The reward for information was recently raised to $500,000.

“Anyone who has lost a loved one wants to be able to put that loved one to rest,” Carson said.

“They want to know that they are finally at peace, the Falconio’s haven’t got that.”