Several recent murders have been carried out by criminals on probation. BBC News looks at how probation works, and how the danger posed by offenders is judged.
Four of Mary-Ann Leneghan's killers were on probation
The murders of wealthy financier John Monckton and teacher Robert Symons, both in London, and the killing of Nottingham jeweller Marian Bates, have had one thing in common - their attackers were on probation.
In the case of Mary-Ann Leneghan, four of her 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 at the time of the killing.
Each year the National Probation Service for England and Wales takes on around 175,000 offenders. The caseload on any given day is in excess of 200,000.
Its cases fall into two areas - offenders newly-released from prison on licence after a sentence of a year of more, or those handed community sentences by the court.
Approximately 70% of offenders supervised by probation officers will be on community sentences, and 30% will be prisoners released on licence to serve the remainder of their term outside prison walls.
1,190 senior probation officers
4,980 probation officers
6,089 probation service officers
Average start salary £21,324 (+ Ldn weighting £3,420)
175,000 offenders begin supervision annually
The caseload on any given day is more than 200,000
SOURCE: Nat Assoc of Probation Officers, Nat Probation Service.
Figures cover England and Wales
According to Cordell Pillay, assistant general secretary of the union Napo, which represents probation officers, an officer's caseload can be up to 70 offenders at any one time.
The NPS will also provide magistrates and judges on request with around 246,000 pre-sentence reports and 20,000 bail information reports every year.
These reports are a guide, giving a number of sentencing options, but the final decision lies with the judge or magistrates.
To compile a pre-sentence report, an offender will be interviewed for between one and two hours. They may also be asked back for a second interview.
An offender can choose to fill in a self-assessment form, which can give a perspective on their criminal behaviour.
A probation officer will also, generally, have access to their criminal record and may consult an offender's social worker. If it is a mental health issue, he or she will refer to their psychologist.
If deemed necessary and appropriate - if, say, they are a "young" 18 - a probation officer can also contact family members.
The Probation Service and the Prison Service assess an offender's risk to the public and to themselves, and to the risk of them re-offending.
Inmates sentenced to four years or more can be considered for early release by the Parole Board, which, in turn, is provided assessments and reports from both the Prison and Probation Services.
If they re-offend or fail to stick to their probation requirements while in the community they can be recalled to prison to serve the duration of their sentence.
Where release is recommended, a "risk management plan" outlining how the offender will be managed in the community and detailing recommended licence conditions, is also submitted.
Those offenders considered to present a high or very high risk are put under Multi-Agency Public Protection Arrangements, which allow agencies - such as probation, police and prisons - to share information and come to a joint view of the risk.
John Monckton was killed by Damien Hanson, on early release for attempted murder.
The "risk" an offender poses while being supervised in the community is assessed as low, medium or high risk using the Offender Assessment System (Oasys).
The Oasys is a questionnaire, the main body of which is comprised of 12 sections designed to reveal the likelihood of re-offending, as well as factors that cause criminal behaviour.
These sections include offending information, accommodation, relationships, emotional well-being and education, training and employment.
Others are the likelihood of peer group pressure, drug and alcohol misuse, and financial management.
This risk assessment is continuous. Offenders can move up or down the register and supervision is adjusted accordingly.
Supervision has to follow the 2005 national standards.
For example, somebody on a community sentence - which can also require an offender to take part in an accredited programme to deal with drug abuse, for example, as well as carry out community service - must meet their probation officer every week for the first 16 weeks - usually for about half-an-hour.
After 16 weeks another Oasys review is carried out and the level of contact therein is decided by the probation officer.
How long an offender is supervised by the Probation Service depends on their sentence. Someone serving a life term will be supervised for the rest of his or her life.