Attacks in Europe planned by ISIS have been in the headlines regularly lately and the threat of ISIS attacks have been in the front mind of national defense operators.
However, these recent attacks were not carried out in a spur of the moment effort or by isolated terrorists. As early as 2012, ISIS has been sending operatives to Europe in preparation for future (now present) attacks. An unfortunate part of this story is that many of these operatives were caught earlier and information was known about their plans. However, no one knew the depth of the organization nor their plans, so little was done to prevent future attacks.
ISIS has been sending operatives in pairs or by themselves for a long time now and it is unknown how many sleeper cells are present in Europe.