A study out of the Brookings Institution used Twitter to shine some light on this, comparing the countries where tweets from ISIS supporters originate. The study dealt with a sample size of 20,000 and found that Saudi Arabia is the top location claimed by Twitter users supporting ISIS in 2015. Syria follows, Iraq rounds off the top three and the U.S. takes fourth place.
The number of fighters joining from Saudi Arabia is between 2,000-2,500, the largest total number, according to the International Centre for the Study of Radicalisation and Political Violence. Per capita, that represents 107 fighters per million people. On a per capita basis, Jordan tops the list, with an estimated 315 fighters per million people.
Belgium has the highest number of fighters per capita of any Western nation. Abdelhamid Abaaoud, a Belgian-born senior Islamic State operative, is suspected to be the key mastermind behind the Paris attacks. He was killed in a raid Wednesday, according to published reports.
France is the biggest source of fighters in Europe, contributing 1,200, or 18 per capita. Government figures have put the number of fighters closer to 1,600.
The U.S. is very low on this list — only about 100 fighters have come from the U.S.
An estimated 1,700 fighters have come from Russia, most of whom are thought to be from Chechnya and Dagestan, according to Russia’s Federal Security Service.