JonBenét Ramsey was killed by her big brother Burke and it was covered up by her parents, investigators have claimed.
The six-year-old beauty queen was found beaten and strangled on the floor of the family's cellar in Boulder, Colorado, on December 26 1996.
No charges have ever been filed in her murder and brother Burke and his parents have always denied any involvement in JonBenét's death.
But a team of experts re-examined evidence in the 20-year-old case, in a two-part CBS documentary The Case Of: JonBenét Ramsey.
In the final minutes of the program, the investigators make the bombshell claim they believe all evidence points to her then nine-year-old brother Burke as being the killer.
However, they said they do not believe it was a case of murder, but said it could have been accidental.
They also revealed Burke had struck his little sister with a golf club after 'losing his temper' and left her with a scar on her face a year before her death.
James Kolar, the former chief investigation for the District Attorney in Boulder, presented the theory all the other experts agreed with.
He believes mom Patsy had left Burke with a bowl of pineapple and some tea and when JonBenét had attempted to sneak some of the fruit later, it prompted Burke to lash out and strike her with a flashlight, killing her.
'My hypothesis was that I think the Ramseys came home at around 9.30, 10 o'clock,' Kolar said.
'I think JonBenét was asleep. I think John did carry her upstairs, Patsy remained downstairs with Burke and served him the tea and the pineapple.
'Then she got JonBenét up to make sure she used the toilet, so she didn't wet the bed that night.'
He added: ' JonBenét was up, she may or may not have brushed her teeth, that stuff was out on the counter, then I think she was up and awake enough that she was maybe still hungry and she went downstairs.’
He added: ‘He probably would have been upset about her trying to snag a piece of pineapple - out of anger he may have struck her with that flashlight.
While the investigators agreed there was no intent to kill, they agree there was clearly an intent to mislead the police investigation.
'The Ramsey family did not want law enforcement to resolve this case and that's why it remains unresolved,' said former FBI profiler Jim Clemente.
He and the other investigators say the lack of the Ramseys getting involved with the case was 'atypical' as most families in such situations are desperate to resolve the case.
'We didn't see that here,' said retired supervisory FBI agent James Fitzgerald. 'But maybe we did. Maybe we saw it with her brother - do everything we can to protect this child.’
They also concluded DNA evidence found on JonBenét's panties which did not match anyone in her family, had no forensic value and that no sexual assault ever took place.
The DNA evidence had been used to rule out the Ramseys as involved in the case, but experts showed how DNA could have been transferred during the manufacturing and packaging process.
In the first part of the documentary, the investigators revealed six extra seconds of audio from a 911 call that appeared to suggest her family were involved.
Using modern technology, they said they uncovered the voices of three people having a conversation after Mrs Ramsey thought she had hung up after calling police 20 years ago.
Three phrases can allegedly be heard during the extra six seconds of 911 audio, which are believed to have come from JonBenét's parents John and Patsy and her older brother Burke.
Investigators believe they can hear Mr Ramsey's voice saying 'we're not speaking to you', before Mrs Ramsey allegedly asks: 'What did you do? Help me, Jesus'.
They claim Burke can then be heard asking 'what did you find?' - even though his parents have always claimed he was asleep at the time.
The documentary aired days after Burke gave his first-ever interview to Dr. Phil.
He said: 'I know people think I did it; that my parents did it. I know that we were suspects.'