The answer does not lie in writing people off."With bitter irony, Friday's attack began at a conference organized by Learning Together, a group that works towards rehabilitating prisoners - Khan took part in the educational scheme, which pairs inmates with people in higher education.
He was released from prison in December 2018 on licence and clearly, a key line of enquiry now is to establish how he came to carry out this attack", Scotland Yard's head of counter terrorism policing assistant commissioner, Neil Basu said on Saturday.
Mr Johnson has vowed to take steps to ensure people are not released early when they commit serious offences. He said it was "repulsive that individuals as unsafe as this man" could be freed.
He said the tragedy raised "enormous questions" about how the attacker's state of mind had been evaluated, and "what supervision and monitoring he was under after coming out" of prison.
Mayor Sadiq Khan said people should "take hope from the heroism of ordinary Londoners and emergency services who ran toward danger, risking their lives to help people they didn't even know".
The attacker was attending the event at Fishmongers' Hall, beside the bridge, and had returned for the afternoon session when he started stabbing people. United Kingdom home secretary Priti Patel, who had recently unveiled plans for tougher sentences for violent criminals, said the police needed "space and time" to complete the investigation and that it was not right to "speculate".
He was later shot dead by police after being accosted by members of the public on London Bridge. Police opened fire after he flashed what looked like a suicide vest.
"It is also by drawing inspiration from Jack and Saskia, who from an early age chose to dedicate themselves to helping others", he added.
The attack has pushed security to the top of the agenda in the election campaign.
He was part of an eight-man network inspired by Al-Qaeda who had plotted to bomb targets including the London Stock Exchange, and planned to take part in "terrorist training" in Pakistan. In 2013, an appeal court quashed Mr. Khan's sentence and replaced it with a 16-year term which allowed for automatic release at the halfway point, under a law adopted in 2008. "It's not lenient policies that are to blame, it's the destruction of the probation service that is supposed to monitor and supervise prisoners after release, & rehabilitation services", he added.
"The solutions to this are complex", he said. "Terrorism has no place in our society and we stand resolute against it". The Daily Telegraph reported Monday that he was featured as a success story on Learning Together's website.
Critics have hit out fiercely at him for appearing to politicise Friday's attack - including the father of the first named victim.
Johnson said the case showed that some criminals are beyond rehabilitation. "But tell you what members of the public just don't do that nowadays", he said. As of yet, the number of those being reviewed is said to be around 70.
"They are being properly invigilated to make sure there is no threat", Johnson told the BBC in an interview.
Khan, who was living in Stafford, was given permission to travel into the heart of London by police and the Probation Service.
News of Jack's death was confirmed on Twitter by his father, David Merritt, who urged that his death not be "used as the pretext for more draconian sentences for detaining people unnecessarily".
Following the attack, authorities are urgently reviewing the release of 74 other people who had been jailed for terrorism and released in similar circumstances to Khan.
West Midlands Police said the man had been "recalled to prison due to a suspected breach of his licence conditions".
One of those injured has been released from hospital, while two are still receiving treatment. The third was discharged over the weekend.