James Bulger detective recalls mother's scream 25 years on

"On the Sunday afternoon myself and other officers were sat in the CID office looking at the CCTV and trying to establish where the two boys were that led James Bulger out of the Strand.

"I heard a terrible sort of scream from the gut, and it was coming from Denise, from where the family of James Bulger were. And then I realised the body had been found.

"There's no doubt - it wasn't an accident."

Phil Roberts, from Powys, was a detective sergeant with Merseyside Police when two-year-old James Bulger was abducted in Merseyside on 12 February 1993.

Grainy CCTV footage showed him being led away from his mother, Denise, by two young boys in the Strand shopping centre in Bootle, near Liverpool.

Two days later he was found dead, having been tortured and killed by Jon Venables and Robert Thompson.

The pair were aged just 10 but they had beaten him with bricks and iron bars, leaving his body on a railway line.

Mr Roberts, from Llanrhaeadr-ym-Mochnant, near Welshpool, arrested and interviewed Thompson - but did not believe someone so young could commit such a horrific murder.

He soon changed his mind.

"We had one witness give a statement to state that one of two boys had blue paint on their shoes.... Blue paint had been found on James Bulger," he said.

"My initial thoughts were 'a 10 year old to commit such an horrendous act, the injuries that were caused on James Bulger? Right, I will negate this boy from my enquiries'.

"It was only as you start interviewing you establish that he was lying, and why was he lying?

"He gave it away by shuffling his legs under the chair. Every time you gave him a hard question, he would shuffle his feet on the chair.

"He eventually started admitting that he and Venables had taken James from the Strand but left him.

"I believe I know why - in so much - that they had full intention of killing somebody, and causing headlines - and they could turn around to somebody and say they were the cause of that."

Thompson and Venables were both convicted of killing James.

They were released in 2001 after eight years in detention, and given new secret identities and and addresses.

Venables has since been jailed again twice for possessing child abuse images - the first time for two years in July 2010 and again last week for three years and four months.

Despite this, Mr Roberts has previously spoken about how he believes Thompson was the ringleader, describing him as "shrewd" and a "vicious person".

He said he was 43 years old with 24 years of service in the police when he arrested the young boy.

"I had seen everything. Nothing had really surprised me," he said.

"This did - and I don't think there could be anything worse than that particular murder."

Mr Roberts said the crime played on his mind, especially when he got home and was with his children.

"You just sit so quiet. You're motionless really," he added.

"My daughters realised that. I think they knew when to keep quiet."

He insists it was important to remember what happened to James and that the age of 10 must remain the age of criminality.

"They knew exactly what they were doing so we have to prevent that," he added.