I suppose this a reply to RJK too.
Policing levels have been cut drastically over the last decade, there is no way you'll see large police presence at events anymore as they can't afford it. Large public gatherings are a terrorist target but many events are now police free.
It would have made no difference the terrorist was letting the bomb off anyway.
I'm not saying they shouldn't but the reality is police and agencies don't have the resource.