A man and woman were killed and three others injured – one critically.
The Friday afternoon attack sent panic through central London and sparked fresh scrutiny over national security just two weeks out from the general election.
The Pakistani-born man was sentenced to eight years in prison in 2012 over a plot to detonate a bomb inside the stock exchange, and target other locations including the US embassy and the home of Johnson, then mayor of London and now Prime Minister. The jihadist and other members of a UK terror cell also planned to build and train at a terrorist military training facility in Kashmir, and discussed launching a Mumbai-style assault on Parliament.
Praising the bravery of members of the public who chased Khan and pinned him to the ground before police arrived, Johnson also suggested the killer should not have been on the streets.
“I have long argued that it is a mistake to allow serious and violent criminals to come out of prison early and it is very important that we get out of that habit and that we reinforce the appropriate sentence for dangerous criminals, and especially terrorists,” he said.
Britain’s top counter-terrorism police officer, Neil Basu, said Khan had been released from prison in December 2018 on parole and “clearly, a key line of enquiry now is to establish how he came to carry out this attack”.
Professor Stephen Toope, the vice-chancellor of the University of Cambridge, said he was “devastated” that an event organised by its Institute of Criminology was targeted in the “hateful” attack.
“We mourn the dead and we hope for a speedy recovery for the injured. Our thoughts are with all their families and friends,” he said.
Metropolitan Police commissioner Cressida Dick said police were working to determine whether Khan had acted alone or was part of a wider network.
“The empty ideology of terror offers nothing but hatred and today I urge everyone to reject that,” she said.
The United Kingdom’s terror alert level was reduced from “severe” to “substantial” in early November, despite 800 live counter-terror investigations and 24 thwarted plots since a deadly assault outside Westminster in March 2017.
The knife attack and the subsequent shooting of Khan sent panic through the surrounding area and forced the evacuation of thousands of shocked office workers and the nearby Borough Market – the scene of a deadly 2017 terrorist rampage.
Australian Prime Minister Scott Morrison paid tribute to the victims and their families on Saturday, and said the government was not aware of any Australians caught up in the London incident or another believed unrelated stabbing attack in The Hague, Netherlands, also on Friday, which left three minors injured.
The judge who sentenced Khan at the time warned he was a “serious jihadist” who should not be released while he and his co-conspirators remained a threat to the public.
“In my judgment, these offenders would remain, even after a lengthy term of imprisonment, of such a significant risk that the public could not be adequately protected by their being managed on licence in the community, subject to conditions, by reference to a preordained release date.”
Khan was initially handed an indeterminate sentence to protect the public but that was overturned on appeal in 2013 and he was ordered to serve at least eight years of a new 16-year fixed sentence.
A report by the independent reviewer of the UK’s anti-terror legislation shows Khan and two other men from Stoke joined a wider terror cell to form plans for attacks on targets including the London Stock Exchange. A target list found by police listed the names and addresses of Johnson, as well as the Dean of St Paul’s Cathedral and the US embassy in London.
Johnson and Labour leader Jeremy Corbyn suspended campaigning on Friday ahead of the general election on December 12. Johnson’s comments about sentencing will spark fresh debate in Britain about how to manage the terror threat as well as the issue of broader crime which is a major issue in the Brexit-dominated campaign.
Police have warned wide cordons will remain in place for several days and said extra armed officers would patrol London streets.
Bevan Shields is the Europe correspondent for The Sydney Morning Herald and The Age.