Jeremy Corbyn called the London Bridge attack “a complete disaster”, and said there are urgent questions to be answered about the role of the Parole Board and probation services.
The Labour leader was speaking in Mytholmroyd, West Yorkshire, on Saturday afternoon, after having stepped back from campaigning following the incident in the capital on Friday.
Mr Corbyn said there needs to be a full investigation into the way all aspects of the Criminal Justice System operates.
He said: “Clearly there has been a complete disaster in that lives have been lost because of his (the terrorist’s) behaviour.
“I think there is also a question about what the Probation Office were doing – were they involved at all – and whether the Parole Board should have been involved in deciding whether or not he should have been allowed to be released from prison in the first place, and also what happened in prison?
“That somebody who clearly was a danger to society… was he given a deradicalisation programme or not?
“I don’t know the answer to those questions, we need to all know the answer to those questions urgently.”
Mr Corbyn added: “I think we have to ensure that the public are safe, that means supervision of prisoners in prison but it also means supervision of ex-prisoners when they are released ahead of the completion of their sentence, to have tough supervision of them to make sure this kind of danger is not played out on the public in future.
He said: “We need to investigate fully the way all aspects of the Criminal Justice System operate.
“Innocent people were killed yesterday – people who were actively doing their very best to try and bring about a more decent and more just society by ending the process of radicalisation and I think we should have huge respect (for) them.”
The Parole Board said it had no involvement in the release of Usman Khan and that he “appears to have been released automatically on licence” halfway through his sentence.
Mr Corbyn will be speaking about the NHS later, and was asked whether he believed the Prime Minister’s reassurances about its future.
He said: “No, I do not believe the Prime Minister one bit because the documents I showed this week indicate that the bottom line for the US is access to public services, crucially access to our National Health Service.
“They put that on the table, they want control of our NHS and control of the process that our NHS pays for medicines in the future.
“So we are not going to do a trade deal with the USA that puts the NHS at risk.”
Asked about the polls and whether Labour had a “mountain to climb”, Mr Corbyn said: “We are doing very well, party members are incredibly active and we suspended campaigning last night and this morning out of respect for those who died at London Bridge, but we are fully on the road and very, very hopeful and very, very confident.
“Our message is ending austerity, ending inequality and injustice in Britain and giving a real chance to everybody, particularly for the next generation.”