It's the murder that gripped the world, and a mystery that has stretched almost 20 years.

On Boxing Day 1996, JonBenet Ramsey was found dead in the basement of the family's home in Colorado several hours after her mother had called police to say her daughter was kidnapped.

The call made by Patsy Ramsey explained that she had found a ransom note demanding money for her daughter's return.

Despite two decades of evidence and investigations, JonBenet's murderer has never been brought to justice, sparking a reel of investigative pieces and documentaries around the mystery.

Burke Ramsey, right, with JonBenet, and parents John and Patricia. Photo / Supplied
Burke Ramsey, right, with JonBenet, and parents John and Patricia. Photo / Supplied

In the first trailer of the CBS documentary titled The Case of JonBenet Ramsey, which will be aired on September 18,investigators revisit the mystery, with particular focus on the 911 call made by the six-year-old's mother when the ransom not was discovered.

The audio of the call shows a highly distressed voice reporting the kidnapping, and demanding authorities to "hurry".

"We have a kidnapping," a frantic Mrs Ramsey says down the line.

"Hurry please."

The operator interjects, asking Mrs Ramsey to explain what the situation was.

"There's a note left and our daughter's gone" Mrs Ramsey replies.

While the call is nothing new to investigators, the operator who received the call is.

The woman on the other end of the line, who remains unnamed in the trailer, admits that in 20 years since JonBenet's disappearance - she has never been questioned by authorities.

"I think it would have really turned the case around," she admits.

FBI investigators who worked on the original case, along with New York City Prosecutor Jim Clemente and Scotland Yard behavioural analyst Laura Richards, hope to bring new light into murder of JonBenet Ramsay through this documentary.

The six-hour series uses new technology and advanced forensics to delve deeper into the murder mystery.

As part of the documentary, CBS completely rebuilt the Ramsey family home - down to the toys and crime scene tape - for investigators to re-enter the scene of the crime.

"We want to get the truth out, so that JonBenet can rest in peace," one of the investigators says in the trailer.

On the day JonBenet was found by her parents, the six-year-old's body was covered with a blanket with a cord around her neck, her wrists bound and duct tape over her mouth. A post mortem showed she had been strangled and had suffered a skull fracture.

Initially, her parents and her older brother Burke were suspects but were cleared in 2008 because of DNA results.

Burke will now be part of a separate sit down with Dr Phil, to mark the anniversary of his younger sister's death. More than 140 people have been investigated as possible suspects in the murder.

Along with the CBS documentary and Dr Phil's interview with Burke - which are both airing in September, Lifetime will also air a film about the movie in November.

The Case of JonBenet Ramsey premieres in Australia on September 18.