In France, officials say, around 1,700 people went to Iraq and Syria since 2013 to join IS. Of those, 400 to 450 have been killed, and 250 returned to France.

Foreign Affairs Minister Jean-Yves Le Drian said on December 8 that about 500 are still in the Iraq-Syria theater, and for them it is now very hard to return to France.

But that leaves another 500 whose whereabouts are unknown, many of them with the skills of war, wielding weapons and making bombs.