LUHE,
The american born ( muslim, legal immigrant parents) perpetrator of the attacks in TN had just been to Syria, twice, before he attacked. Traveling to Syria and his Facebook page showed signs of extremism. Yet this young man grew up here, He went to school with my own kids friends, He lived in the same neighborhood as my daughter .( It was surreal, and frightening as he;l to hear my daughter's street address reported on national news on my car radio, as being surrounded by SWAT teams) Unless people traveling to the countries on the danger list for extremism training are detained, or denied entrance until they can be considered safe ( How do you do that? I don't know. Just this young mans facebook page combined with his trips could have been enough to not allow him back in the US. ) Then this type of threat/dangerous person is freely allowed to re-enter.
This is the type of person who does need to be kept out of the borders of the US ( and from the UK, out of those borders) The signs were there in the months before the horrid jihadist terrorist attack