JonBenet murder mystery: FBI agent reveals the investigation's biggest flaw

The first FBI agent to arrive on the scene at the Colorado home of JonBenet Ramsey after she was reported missing has detailed the errors made by police in the doomed case.

With the mystery still unsolved 20 years later, Ron Walker said the biggest flaw was failing to interview JonBenet’s parents, John and Patsy Ramsey, separately.

“[People have] criticised the Boulder police for not doing what the police should have done on the 26th of December, and that was separate John and Patsy into two different interview locations, and get them interviewed, and get a full signed statement from them,” Mr Walker said in a new US documentary Overkill: The Unsolved Murder of JonBenet.

“Everybody makes mistakes at crime scenes, and I don’t want to lay the blame on any particular person, but it was the philosophy in the police department at that time on the command staff that inhibited the officers and the detectives from doing the job that they knew they needed to do.”

Mr Walker’s comments come as Colorado investigators announced that they will conduct new tests on DNA evidence surrounding the child beauty queen’s death.

"This might give us new information that could be helpful to the investigation," District Attorney Stan Garnett said.

According to reports, items that will be tested include the underwear and long johns that the six-year-old was wearing when she was killed.

Mr Garnett added that authorities do not expect DNA test results alone to "definitively solve or prove the case".

As the 20th anniversary of the murder mystery approaches, a handwriting expert has claimed the ransom note found at the Ramsey home was likely written by JonBenet’s mother.

“It’s highly probable that she wrote the ransom note,” expert Cina Wong told ABC’s 20/20.

Ms Wong said she spent three weeks examining 100 examples of Patsy Ramsey’s writing and claimed to have found over 200 substantial similarities between the mother's handwriting samples and the ransom note.

Ms Wong said the grouping of certain letters such as ‘t’ and ‘e’ were evident in both writings, as well as misplaced capital letters and the same variation of the letter ‘a’.

Mrs Ramsey, who died in 2006, strongly denied having any involvement in her daughter’s death.