Prevent ain't Preventing

So, the perpetrator of the latest violent act against the Jewish community (finally recognised as a terror attack), was yet another person referred to prevent. 

How many times do we have to hear the phrase "known to the authorities" or "Referred to Prevent".

Prevent, as an organisation is supposed to de-radicalise the people referred to it. Now, I'm not sure how many have been successfully de-radicalised, but it seems there are a great many failures of the prevent strategy.

I just have to ask, what happens to prevent's failures? Are they just left to enter society and be ticking time bombs? 

I would like someone to be asking teh question, because it seems quite clear there is a whole raft of radicalised people out in the community that pose a risk to that community. 

There must be a further step for anyone that is referred to prevent and who fails to be de-radicalised. What that step is for the authorities to decide, because it's only them who have the power to remove the liberty of people. Because is that the next step incarceration? It seems it should be, given the risk these people pose.