Chilling audio tapes reveal the moment two ten year old boys admit to the gruesome murder of a toddler.

The heinous way this young child was tortured for hours and then murdered still haunts people all over the world. The fact this crime was perpetrated by two young boys is unsettling in the extreme.

In February 1993, when Detective Inspector Jim Fitzsimmons from Merseyside Police stared into the seemingly innocent eyes of 10-year-olds, Jon Venables and Robert Thompson, he couldn't comprehend the evil the pair were capable of, and what harm they inflicted on their tiny victim.

Today, almost 30 years on from the abhorrent murder, many of Inspector Fitzsimmons colleagues who worked on the case, are also traumatised by the disturbing details of the crime.

And for them, listening back to the police interviews with Thompson and Venables is as chilling now as it was when they were first recorded.

For Detective Sergeant Phil Roberts, the most sickening response came from Robert Thompson when he asked the boy what James' final words were.

The 10-year-old killer responded by mimicking the toddler's voice.

"I want my mu-um," Thompson repeated.

"There's no way I could relate to a child actually committing a murder," Sergeant Roberts said.

It's a view shared by millions of people everywhere: Just how could two ten-year-old boys snatch a two-year-old toddler away from his mother in a busy shopping centre, and then savagely torture him to death?

Sunday on 60 MINUTES, inside the investigation of the crime that continues to shock the world.