When was Khan jailed and for how long?
Khan was given an open-ended jail term – known as an 'imprisonment for public protection', or IPP – in January 2012 at Woolwich Crown Court after pleading guilty to one count of 'engaging in conduct in preparation for acts of terrorism'. The sentencing judge Mr Justice Wilkie specified a minimum custodial term of eight years. But to secure his freedom, Khan would have to convince the Parole Board that he no longer posed a risk.
What happened then?
In an appeal in March 2013, Khan's lawyers won their case – and he was given a term with a definitive end point. The need for Khan's release to be approved by the Parole Board was also dropped. Appeal judges imposed an extended sentence of 21 years which comprised a custodial element of 16 years and a five-year 'extension period'. The 16-year custodial element meant he was eligible for release at the halfway point – eight years.
Why is only half of a sentence served?
It has been a convention since the 1960s that half of a term is served in prisons. The rest of a sentence is served 'on licence', when an offender can be quickly sent back to jail if they fail to behave.
When was Khan finally freed?
The Parole Board was quick to point out after Friday's attack that Khan's release was not referred to them - he was automatically released at the halfway point. He remained on 'extended licence' and had to report to police and probation officers, wear a GPS electronic tag and fulfil other requirements.
How did laws passed by a former Labour government affect the Court of Appeal's options?
PM Boris Johnson has said Khan had to be 'automatically released half-way through' because of changes Labour made in 2008 to Extended Sentences for Public Protection or EPPs. This is correct.
Until 2008, anyone on an EPP had to have their release approved by the Parole Board. If they were refused, the board could keep them in jail up to the end of their custodial period, which in Khan's case was 16 years.
But in mid-2008, Labour made release automatic halfway through.However, the Court of Appeal could potentially have upheld the original IPP sentence.
How can ministers toughen up the sentencing of terrorists?
Khan's atrocity has reignited debate over whether there is now a case to remove entitlement to early release for convicted terrorists.
PM Boris Johnson has already said they should be made to serve 'every day' of their terms. Some important steps have already been taken.
Extended Determinate Sentences (EDS), brought in in 2012, only allow convicted terrorists to apply for parole two-thirds through their sentence, with no automatic entitlement for release.
The Counter Terrorism and Border Security Act, which won Royal Assent in February, toughens jail terms for a range of offences and – crucially –makes it easier to keep terror suspects behind bars beyond the halfway point. It extended two types of sentence – the EDS and Sentences for Offenders of Particular Concern (SOPC) – to a number of middle-ranking terror offences.
A clearer structure could set out underlining principles such as whether early release is allowed, and whether the Parole Board or ministers should approve any release before it takes place rather than it taking place automatically.
A clearer structure would help underline how the justice system should deal with terrorists.