Perhaps the most likely avenue for such a plan depends on compromise at least one person in the passenger screening process. If you know that they have a plan

Perhaps the most likely avenue for such a plan depends on compromise at least one person in the passenger screening process. If you know that they have a plan which has a reasonable probability of working given a compromised person working the carry-on scanner and you know the airports where these groups are most likely to compromise such individuals then it seems logical to start with those locations. Terror groups can get into Europe, but do they have people working airport security a CDG or FRA?