U.K. Prime Minister Boris Johnson says the man who killed two people near the London Bridge on Friday should never have been let out of prison — but a specialist who works to deradicalize extremists says throwing away the key won't solve the problem.

Usman Khan was at a conference on criminal rehabilitation in London on Friday when he went on a deadly stabbing rampage, killing two people and injuring three others, before police shot him dead.

He was convicted for terrorism offences in 2012 for his part in an al-Qaeda-inspired bomb plot, and was released in December 2018, subject to conditions, before the end of his sentence.

In an interview with the BBC's Andrew Marr on Sunday, Johnson blamed the attack on the previous Labour government's early release policies.

But Hanif Qadir, a former adviser to the U.K. government, says Conservative cuts to rehabilitation services are to blame. His own anti-extremism program, the Active Change Foundation, lost its funding under a previous Tory government.

Here is part of his conversation with As It Happens host Carol Off.

What do you make of Boris Johnson's claim that "lefty governments" like Labour are to blame for Usman Khan's early release from prison?

I think it's a shame that Boris Johnson has to come and use a pathetic excuse that he was let out early. Well, look at all the other terrorist convicts that were arrested after ... the Tory government came in, and have been released early.

All these prisoners that go into prison, whether they are terrorists ... or whether they are other violent criminals, you know, if they're given a 10-year sentence, they're going to come out in five years. And under good behaviour, they come out in four years. So there's nothing new here. So what Boris is saying is absolutely nonsense.

He's not talking about the failures in the system that his government have created — the cuts to the resources and probation [programs].

Usman Khan is pictured here in 2012. (EPA-EFE/West Midlands Police)

Could these deaths have been prevented?

Of course they could have.

We don't know ... whether [Khan] was assigned a mentor. And if he was, did the mentor argue with the probation to allow him to come to this conference?

Because he's allowed to come unaccompanied. He's allowed to come carrying knives. Nobody's looked at that. Nobody's thought about that. There was no police on the premises. He was left unattended for five minutes, and look what he did.

A man wearing a fake explosive vest stabbed several people near London Bridge Friday. At least two people were killed. Shockwaves of the attack were sent through nearby Borough Market. That's where Rob Underwood and his wife were eating lunch when the attack happened. 6:31

This gathering was called Learning Together. It was a program to bring students from the university and prisons together to discuss justice issues. But at the same time, Mr. Khan ... was radicalized. This is a different kind of person out on license or bail, isn't it? It's not the same thing as other kinds of crimes when someone has been radicalized. You're very familiar with that. How would you know whether he had been de-radicalized or not?

We know that he's radicalized. We know he went to prison for terrorist-related offences. ... We know that prisons, unfortunately, are incubators for radicalization.

So why did we relax our safeguards and our protocols? The fact is, you will never know when the person has changed. I've taken people like him — in fact, much higher risk cases than him — to events like this.

You have to put in measures. You will have to cross-check everything. Make sure he's accompanied. Make sure you check his bags. Make sure when he arrives, that he's not left unaccompanied. Make sure you follow him everywhere.

Vigils were held in London and Cambridge to remember the people killed and injured in last week's stabbing attack on London Bridge. 0:57

Usman Khan's lawyer has said he was completely shocked. What he saw of this man, over the eight years he had been at prison, was that he had changed. The way he talked, his manner, his conversations were completely different. ... So do you think you could have identified that Mr. Khan was a threat?

Look, it's not as easy as what I'm saying. It is challenging.

But the lawyer is a lawyer. He's an expert in his field. He is not an expert in understanding the ideology, the nature of these kinds of individuals. I've worked with hundreds of people like this and I know how deviant they can be.

I've worked with people who we've mentored, and we've moved forward, and they're back in the mainstream. But I've also worked with similar kinds of individuals who haven't changed, who've been very clever, and been very crafty, and been very devious, who we've had to send back to prison because I wasn't prepared to take any further risks with these guys.

So with the probation system that we had in place when the resources were there ... before Boris Johnson's government took away those resources, we made sure that if the person is not changing, the best place is for them to go back to prison.

Should they be indefinitely locked up, which is what Mr. Johnson is suggesting?

I appreciate that there are people that will not change, and some of them need to be locked up indefinitely. I agree with that.

But when you're working with individuals that are going to be released — and that's the way the criminal justice system works here — you have to have the right kind of people in place.

What's happened with Boris Johnson's government is they've been trying to fix something that's not broken. It was working. You know, we were celebrating the fact that the U.K. had one of the best models in the world in preventative and rehabilitation.

But trying to ... reduce the resources has made things worse, and we've seen that from what's happened Friday.

Police identified Saskia Jones, right, and Jack Merritt as those killed in Friday's stabbing attack near London Bridge. (Metropolitan Police Service/The Associated Press)

One of those victims, Jack Merritt, his family is saying that if Jack were alive, he'd be livid at the thought that his death would be used to change things, to perpetuate an agenda or to lock people up and throw away the key. ... Do you think, given the fact that you're in the middle of a very charged election right now, that it's likely this will be used politically?

It's very unfortunate that that's what's going to happen.

We saw Boris Johnson and the Home Secretary Priti Patel very quick to ... lay the blame on other political parties or the other governments without actually looking inwards and looking at their own mistakes and their own lack of safeguarding and protocols.

Written by Chris Harbord and Sheena Goodyear with files from Reuters and Jeanne Armstrong. Interview produced by Chris Harbord. Q&A has been edited for length and clarity.