Lead investigator on the JonBenet Ramsay case comments on her brother Burke's bizarre smile.

The mysterious death of six-year-old beauty queen JonBenet Ramsey has had the world transfixed for over 20 years.

The case has grown such notoriety, that a team of investigators have re-opened the murder file for a television series that hopes to solve the mystery behind her untimely death.

Today show hosts Karl Stefanovic and Lisa Wilkinson invited one of the lead investigators involved, retired FBI agent Jim Clemente, to speak about the show that will premiere on Monday night.

Stefanovic asked Clemente what he thought of the “very odd interview” JonBenet’s brother gave to Dr. Phil last week.

Clemente said the team looked at the brother throughout the investigation and that it would be unwise to judge his character from one interview.

"We look in detail at the tapes that were made - the videos that were made of him - over the course of four different days, a year and a half apart and that's going to tell you a lot more about his personality," he said.

Co-host Wilkinson asked about the ransom note found by JonBenet's mother Patsy Ramsey.

"What can you tell us about that letter?" She asked.

Clemente said the letter was a historical and very telling piece of evidence.

"There has never been a case like this before or since - where there is a three page ransom letter left and the body of the victim who was supposedly kidnapped," he said. "It's just a complete, bizarre circumstance."

The investigator said he and the team had worked collaboratively to assess each and every piece of evidence before they reached a conclusion.

"The whole point of this documentary series is to build this brick-by-brick-by-brick," he said. "Together we build the comprehensive foundation for the comprehensive theory."

Clemente said the experts would discuss how the injuries occurred, the DNA evidence behind it, the behaviour of those involved and the linguistics used in given statements.

The interview ended when Wilkinson asked whether he believed authorities had overlooked certain aspects of the crime.

"Do you think police missed things at the time?" She said.

The retired FBI agent said there was a crucial window missed by investigators when the crime was still considered a kidnapping.

"Well yeah, the case started out as a kidnapping, remember - that means you have to investigate outside the home but then it shortly after - eight hours later - it became a local homicide, so now you have to investigate inside the home," he said.

"And that made them miss eight hours of time where the crime scene was actually trampled and contaminated."

The documentary series The Case of: JonBenet Ramsey is set to premiere on Monday the 19th of September on Channel Nine.