Now the questions have to be asked.
To honour the victims of brutal terror, the injured and the bravery of public heroes, police and emergency teams.
And to protect Britain’s streets from murderous evil.
Our thoughts are first and foremost with the innocent people killed and injured in the London Bridge attack.
But we also need to know why a convicted terrorist can be free to murder even while under supervision by the authorities.
Two questions are paramount.
First the sentencing of convicted terrorists and the process for deciding when they are safe to be released.
Secondly the system for deciding whether a prisoner has truly been deradicalised – disconnected from their warped, ideological mission to kill in order to divide, disrupt and demoralise our way of life.
The fact that Usman Khan was able to strike was a failure. But not a failure of the security and intelligence services, which have thwarted 22 terror attacks in the past two years.
Khan was in a progamme designed to assist prisoners wanting to leave their indoctrination behind and ease their way back into society.
It is tragically clear he had not changed. There are 218 terrorist prisoners in the same category as Khan. In the 12 months to March this year 51 inmates with terror links were let out of jail.
To what extent can we be sure that convicted terrorists in the same category as Khan have genuinely been deradicalised?
The authorities need to explain.
Khan, 28, was released just under a year ago under rules which allowed him to serve only half his 16-year sentence.
New laws mean that the Parole Board now decides when such offenders can be released. He slipped through the net.
If no warning signals were detected we need to know why not. And if they did spot any danger signs, what did they do about it?
Police Commissioner Cressida Dick got it right when she said yesterday: “We have seen the worst in human kind, but we also saw the very best of human spirit.”
To honour that, we need the answers.
