Gang members who tortured and killed Mary-Ann Leneghan while on probation were not considered high-risk, a senior probation official has said.
Mary-Ann's killing again puts the spotlight on probation services
Their previous offences "did not appear to warrant intensive supervision", Probation Boards Association chief executive Martin Wargent told BBC News.
Home Secretary Charles Clarke has said it is vital for the probation service that lessons are learned from the case.
But Mr Wargent insisted: "There is no fail-safe system."
At any one time the service could be supervising 250,000 offenders, he said.
"It is not as if we are dealing with a small group of well-behaved people - these are people that have been identified as the most likely to commit more offences," he told BBC Radio 4's Today programme.
"Some people out of a total like that are going to re-offend.
Mr Wargent added: "Resources are given to different cases according to the level of risk.
"Most crime in this country does not even end up in court.
"Only about two cases in every hundred offences actually result in a criminal conviction.
"But even given that, one man in three in this country has a significant criminal conviction.
"That is a frightening fact.
"That is the size of the problem."
The service's success rate among "high-risk offenders" was "extraordinarily good", Mr Wargent told Today.
"Last year we had 13,000 very high risk offenders - only 0.6% re-offended."
The Conservative MP for Reading East, Robert Wilson is calling for a public inquiry into the probation system.
"We need an inquiry to look at, firstly, the system - whether it's structured right, whether it's resourced right, whether it can cope with what's been placed upon it."
A spokesperson for London Probation said that there were no concerns about how they had managed the four men's cases, and that they would not be carrying out a review of services as a result of what had occurred.
"It is our belief that all four of these cases were managed to national standards. In a case such as this, where people under supervision are involved, there is always an automatic review of the case management.
"We are already in the process of carrying out a root and branch review of the service, and have been recruiting extra trainee probation officers. There will not be a separate audit on the basis of this case."
Shadow home secretary David Davis has called on ministers to realise "the shocking consequences of their policy of allowing dangerous criminals out of prison, and to own up to their complete mismanagement of the probation service."
Four of Mary-Ann's killers - drug-dealer Adrian Thomas, 20, of Battersea, and fellow gang-members Michael Johnson, 19, of Southfields, Jamaile Morally, 22, of Balham, and Indrit Krasniqi, 18, of Chiswick - were under supervision in the community.
The director of the Jill Dando Institute of Crime Science, Professor Gloria Laycock, told BBC News it would have been "almost impossible" to predict how dangerous they really were.
"The service are supervising thousands of young people in the community - many of them with records of violence - and to suggest they can predict, or should have somehow known, these young men would do such an absolutely appalling offence is just simply not fair.
"Because of the enthusiasm for sentencing in the community, that figure is going up.
"And it is fair enough to ask questions about whether the service is properly resourced and can cope with those increasing numbers.
"But even if we had the very best, well-resourced, well-trained probation service on the planet, these things will still happen."
Ms Laycock added: "The probation service only sees these people twice a week... that leaves an awful lot of time for them to do other things.
"It is not sensible to expect the probation service to anticipate and predict that kind of behaviour.
"It just cannot happen."
The Leneghan case will be studied as part of Home Secretary Charles Clarke's review of the probation system.
He is due to report to Commons on changes needed in April.