Jon Venables, 27, who was given a new identity when he was released in 2001, was taken back into custody after an unspecified breach of his parole conditions, a Justice Ministry spokeswoman said Wednesday.
She refused to describe the nature of the breach, where it occurred, or exactly when it happened, saying only that it was a recent development. She spoke anonymously in line with department policy.
But criminal justice experts quoted in Britain's media said Venables' parole violation must have been serious or repeated if he was recalled to prison.
"It might be a technical matter, like missed appointments, but I can't see that happening in this case," Harry Fletcher, assistant general-secretary of Britain's probation officers' association, told Sky News television.
Venables and his friend Robert Thompson were only 10 when they abducted 2-year-old James Bulger from a shopping center in Liverpool as he waited outside a butcher's shop for his mother. The boys dragged the toddler through town to a railway line, where they hit him with bricks and metal bars, poured paint in his eyes and finally placed him on the tracks, where a train cut him in half.
A security camera captured images of the toddler being led away by the two older boys, scenes replayed countless times on British television and around the world.
Venables and Thompson spent eight years in a secure children's unit after being convicted of abduction and murder. But they were released in 2001, only a few weeks before their likely transfer to an adult prison.
The murder repulsed the country and the decision to release the killers — their anonymity protected by court order — divided opinion in Britain. Fletcher told Sky that the news was a blow to prison officials, who invested an enormous amount of energy into trying to reform the killers.
"There would have been intense disappointment because a lot of work, a lot of energy, went into rehabilitation here," he said.
Denise Fergus, Bulger's mother, said at the time that she was disgusted with the government. On her Twitter account Tuesday night she wrote: "would like to let everyone know Jon Venables is were he belongs tonight, behind bars."
As a condition of their release, Venables and Thompson are not permitted to contact each other or any member of James' family. They are also not allowed to enter the Liverpool area in northern England without permission from probation officials.
They can be recalled to custody at any time if officials find evidence that they present a risk to the public.