As Europe reels from recent terrorist attacks, citizens who left the continent to join armed struggles in the Middle East are returning home—indicating possible security concerns for their countries of origin.

According to a study released by the Hague-based International Centre for Counter-Terrorism on Friday, of the up to 4,294 “foreign fighters” that have left the European Union to join the various rebel ranks in Syria’s civil war, an average of 30 percent of these soldiers have returned to their home countries. Around 2,838 of them hail from just four countries: Belgium, France, Germany and the United Kingdom, with Belgium being the largest per-capita contributor fighters in Syria.

But of these four major sources of foreign fighters, large discrepancies arise. For example, Belgium, the site of last month’s deadly bomb attacks on an airport and metro station, sent 41 fighters per million population—the most among any European country. However, only 18 percent of those fighters returned home. Compare that to the 50 percent who returned home from Denmark.

In total, France was the largest source of foreign fighters, counting more than 900. The country’s return rate mirrored more closely the continent’s average, with 27 percent coming home.

The study stresses that not all foreign fighters are terrorists, and not all terrorists are foreign fighters. After all, the figures do not describe a homogeneous bunch: of the total number of foreign fighters, 17 percent are female, and between 6 and 23 percent are converts to Islam. That being said, almost all of them come from urban areas.

The authors still maintain that the EU should take its security situation very seriously and bolster policies that “seem to be lacking” among the union and its member states. Over the past 18 months, Europeans who fought in the Middle East have been involved attacks in Paris and Brussels.

“Considering that many terrorist organizations aim to undermine tolerant and pluriform, rule-of-law based societies, this risk should not be ignored,” the report says.