The San Bernardino, Calif., massacre brought even more attention to Islamic State’s frighteningly rapid global growth, and where around the world it has found traction — not just in terms of territory gained, but in support.

Related: How much money goes to fighting and funding ISIS, in two charts

Even with many details of the case still unknown, law-enforcement officials see a chilling terror danger from extremist sympathizers who, unnoticed by authorities, are able to amass deadly arsenals to attack vulnerable gatherings anywhere in the U.S., writes the Wall Street Journal.

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 foreigners joining the conflict in Syria and Iraq has continued to rise in 2015, though this data doesn’t track these fighters to Islamic State, specifically. Nonetheless, the data paints a rough picture of where around the world ISIS is finding success in recruitment.

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, was suspected to be the key mastermind behind the coordinated attacks on Paris in November. He was killed in a raid at an apartment building in a Paris suburb six days following the attacks.

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. Islamic State has said it brought down a Russian passenger plane in Egypt’s Sinai region in late October with an improvised explosive device, killing all 224 passengers on board.