HIDDEN IN PLAIN SIGHT - SMH

Many letters this week have posed the question as to why Monis was out and about on bail despite everything the police and courts knew about him. Meanwhile an article in Friday's Herald informs us that Abedi had been reported to the authorities on several occasions as a person to watch including a warning from his local mosque. After 9/11 we learned that authorities in America had information about the suspicious behaviour of the men who flew the planes into the WTC. After the terrorist incidents in France security services moved swiftly to round up perpetrators and their known associates.

What is going on here? 
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The men who commit these acts of violence often seem to be "on the radar" but no preemptive action is taken to get them out of circulation before deaths and injury occur to innocent people.
I'm wondering why that is.