The haunting true story behind the popular saying, "Don't take candy from strangers".

It was July 1, 1874. Six-year-old Walter Brewster Ross took his four-year-old brother, Charley, to play outside their home in the leafy, affluent suburb of Germantown, Philadelphia.

As they frolicked on their front lawn, a black buggy rolled up with two familiar men inside. The boys had spoken to them just days earlier; they’d given the brothers lollies and offered to take them for a ride. But Walter and Charlie had refused that day, remembering their father’s warnings not to accept gifts from people they didn’t know.

But on this summer afternoon, the men offered something irresistible, especially with Independence Day approaching. Fireworks.

The boys went in the buggy to a local store, where Walter was handed 25c and sent in to buy the crackers. When he returned the men and the vehicle were gone. Charley was nowhere to be seen.

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The case of little Charley Ross, one of the most famous missing persons cases in American history, is believed to be the origin of that cautioning urge, “Don’t take candy from strangers”.

It’s also recognised as the country’s first ransom kidnapping.

Two days after Charley vanished, a letter arrived at his father Christian’s drygoods store in downtown Philadelphia, according to The Smithsonian. It was scrawled in black ink, the writing appeared unsteady and was riddled with errors.

“Mr Ros, be not uneasy, you son charley bruster be all writ we is got him and no powers on earth can deliver out of our hand. You wil have to pay us before you git him from us, and pay us a big cent to [sic],” the note read. “if you put the cops hunting for him you is only defeeting yu own end.”

A second note followed three days later, this time with a sum, “This is the lever that moved the rock that hides him from yu $20,000. Not one doler les — impossible — impossible — you cannot get him without it.

“If yu love money more than child yu be its murderer not us.”

The demand was roughly the equivalent of US$400,000 today, or AUD$553,000.

Though the Ross family lived in a wealthy neighbourhood, they were of modest means. Christian had been a victim of a significant stock market crash ten years earlier, and did not have the money to pay his son's kidnappers.

Instead he took the letters to police. Charley's face appeared on posters throughout the region, and his disappearance soon became national news after the mayor's office announced a $20,000 reward for his safe return.

A total of 23 letters arrived over five months, according to The Smithsonian, but still no sign of Charley.

The suspects.

On December 13 that year two career criminals, Bill Mosher and Joe Douglas, were shot at the New York home of a judge, their robbery attempt foiled by armed residents, according to The Pittsburgh Post Gazette.

Though Mosher died instantly, Douglas clung to life for two more hours, during which time he confessed that he and his criminal partner were responsible for the abduction of Charley Ross. The poverty-stricken father of four didn't provide any details about the crime and claimed only Mosher knew the little boy's whereabouts.

Douglas' wife substantiated his claims during a police interview, according to The Milwaukee Journal, and when Walter was later taken to a New York morgue to identify their bodies he confirmed that they were the men from the black buggy.

A desperate family's search.

Christian Ross and his wife, Mary Ann, spent the rest of their lives and money looking for their son.

Speaking at the 1875 trial of William Westervelt, an associate of Douglas and Mosher who was ultimately convicted as an accomplice in Charley's kidnapping despite maintaining his innocence, the grieving father estimated that more than 500,000 people across the country had been involved in the search for Charley, that 700,000 flyers had been distributed and that more 600 children who resembled the boy had been investigated.

Over the coming decades, several men came forward claiming to be Charley Ross. The family dismissed all as impostors.

Christian died in 1897, aged 73, and Sarah in 1912, at age 79. Their Charley was never found.

But a century later, in the attic of a Philadelphia home, the ransom letters were. They were piled neatly among a collection of things left to the homeowner, Bridget Flynn, by her grandmother, according to The Smithsonian. It's not known how they had come into the deceased woman's possession - it seems likely her forebears had purchased them in an auction. But in 2013, they were sold to an unidentified local collector. The winning total? The same as that demanded by Charley's kidnappers in 1874.

$20,000.