A lot of them, Harry, are British nationals and not even naturalised ones at that (from whom nationality could be stripped).
There is a threshold beyond which the authorities can intervene e.g. if there is evidence of collecting information (even visiting web sites).
This requires heavy surveillance which is why we live in a Big Brother society nowadays and universal monitoring of us all is no longer something just in a George Orwell novel.
The youngest victim who was killed has been revealed to have been EIGHT years old. EIGHT!!! How could she ever be a "crusader"?!
The reaction of some has been an outpouring of hatred but I am pleased to report that the Manchester Evening News paper set up a fund for the victims that has, within hours, raised more than £473,000 ($614,000). If anyone wants to give Manchester some love in a financial way, the page is here:
https://www.justgiving.com/crowdfunding/westandtogethermanchester
Please, Manchester members, check in and let us know you and your loved ones are OK. Praying and thinking of you all.