The mysterious phone call before JonBenet Ramsey's murder that we're only just hearing about.

As the 20-year anniversary of JonBenet Ramsey’s death nears, more theories about who murdered the six-year-old are continuing to emerge.

Mystery has surrounded the pageant queen’s suspicious disappearance and death since she was dead in the basement of her family’s Boulder, Colorado home on Boxing Day 1996.

Now bestselling authors Lisa Wilson and Nick Van der Leek have revealed that an ominous warning came just three days before the murder when the Boulder police received a silent emergency 911 call.

Concerned about the silence at the other end of the line, police called the phone number back. When no one answered, they traced the number to its address - the Ramsay's palatial home - where a party was taking place.

Police were greeted by the family's friend, Susan Stine, who would not allow them into the house and blamed the emergency call on someone dialling the wrong number while trying to order medication for an elderly parent.

“We believe something happened at the party, something serious,” the authors told news.com.au

“Secrets kept on December 23 were repeated under far more serious circumstances three days later," they continued. "This time the Ramseys knew they could rely on their friends to keep their secrets. Their friends assumed the Ramseys were innocent, and if they felt otherwise later, they were ostracised, sued or accused.”

Concerns for JonBenet were raised early on Boxing Day morning, when her mother, Patsy, awoke to find a three-page ransom note from "a small foreign faction" demanding $118,000 in exchange for their youngest child, who they claimed to have kidnapped.

Patsy's first response was to phone the Boulder police, and then the same friends who had been at the house on 23 December.

JonBenet's then nine-year-old brother Burke Ramsey was also in the house at the time but has long claimed not to have heard or seen anything.

Hours later, the little girl's body was discovered in the basement with a homemade garotte around her neck and a fatal blow to her head.

The family has maintained they believe someone broke into the house and killed their daughter - claims that were largely disproven in the recent two-part CBS documentary.

“JonBenet had quite a lot of damage to her body,” Mr Van der Leek continued, explaining, “Her back showed scrape marks that suggest she was lying on her back, the garotting happening after she turned onto her stomach. I don’t really see it was abuse or assault, it was something else — interference.”

As for the family friends - Wilson and Can der Leek believe they know more in the still unsolved murder than they may have previously let on.

“There was this incredible solidarity with them,” Wilson said.

“What’s frightening about the case is the way a very wealthy family got everybody to dance,” Mr Van der Leek added, noting that any families who have spoken out in the two decades that have passed have been sued or viciously slandered.

No one has ever been charged with the murder of JonBenet Ramsey.