UK citizens are poised to vote on a crucial decision about whether to stay or leave the European Union (EU) -- but the debate has been stalked by the fear that one big terror attack in the UK before Thursday's poll could influence the outcome.

So far in the referendum campaign, Britain -- although traumatized by the brutal murder of British lawmaker Jo Cox -- has fortunately only been  a spectator to the terrorist scourge of radical Islam. In Paris the brutal stabbing of a police officer and his partner (also a policewoman) in their house left their three-year-old son orphaned. Terrible enough, but the knife-wielding attacker also broadcast his actions live.

As Europe awoke to this new level of terror -- the potential of mass execution being streamed live -- the U.S. was experiencing its own horror with dozens slaughtered in the Orlando club shooting.

Read more: CNN