The home secretary Priti Patel has blamed a previous Labour government for the release of the London Bridge attacker who had been deemed dangerous, as the parties rowed over who was responsible.
Jeremy Corbyn, the Labour leader, said questions need to be urgently answered over the roles of the Parole Board and probation services.
But Ms Patel said the Parole Board had not been involved in the decision to free Usman Khan, who killed two people and wounded others in a knife attack on the bridge on Friday.
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Khan was freed from jail on licence last December, having served less than seven years of his sentence.
Ms Patel, who had earlier visited the scene of the attack with the prime minister, posted a link to a news article and tweeted: “The Parole Board could not be involved in this decision, jeremycorbyn.
“Your party changed the law in 2008 so that Khan was automatically released irrespective of the danger he posed. Very concerning that you want to be PM but don’t understand this.”
The Parole Board has said it had no involvement in his release and that Khan “appears to have been released automatically on licence” halfway through his sentence.
Khan and two others were originally given indeterminate sentences with a minimum term of eight years behind bars but on appeal the indeterminate sentences were dropped in 2013, making him eligible for release under licence.
Labour’s Yvette Cooper, chair of the home affairs select committee, had tweeted questions about Khan’s release, saying: “Usman Khan was sentenced for serious terror offence in Feb 2012. Thought to be so dangerous by judge he was given IPP sentence to prevent release if still serious threat.
“Instead he was released 6 yrs later without Parole Board assessment. How cd this be allowed to happen?”
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Ms Patel responded: “Because legislation brought in by your government in 2008 meant that dangerous terrorists had to automatically be released after half of their jail term.
“Conservatives changed the law in 2012 to end your automatic release policy but Khan was convicted before this.”
Ms Cooper said there would need to be a “serious and full” investigation into how the incident happened given that the attacker was a convicted terrorist.
She told BBC News questions needed to be asked, particularly because there may be other people in the same situation as this attacker.
“There are deradicalisation programmes but there is a question about whether this attacker was a part of them, what the extent of them was, what the resourcing is behind them,” she said.
Sentences of Imprisonment for Public Protection (IPPs) were introduced in 2005 and given to violent or sexual offenders who posed a risk to society.
They would not include a fixed term of imprisonment but instead would give an offender a minimum period to serve before they could be released.
A prisoner serving an indeterminate sentence would stay in prison until it was found they were safe to be released, meaning they could remain locked up for decades if they were still thought to be dangerous.
They were abolished in December 2012, but only for new prisoners.
It’s thought Khan was automatically released thanks to the Criminal Justice Act – introduced by Labour in 2005 – which lets out prisoners halfway through their term, with the rest of their sentence under licence.