Almost 30 percent of EU citizens who joined the fight in Syria have returned home, according to a new study.

The study, prepared by the Hague-based International Centre for Counter-Terrorism, said that more than 4,000 Europeans had gone to fight in Syria, of whom 14 percent were confirmed dead.

French, Germans and Britons make up the highest number of foreign fighters from European countries in the ranks of armed groups in Syria, the study reported, but Belgium is the largest contributor in proportion to its population.

Source: The International Centre for Counter-Terrorism

Europeans fighting alongside groups in Syria and Iraq have been high on the agenda of European security concerns for several years.

Returned volunteers have been involved in attacks in Paris and Brussels over the past 18 months, including last month's bomb blasts in the Belgian capital.

The study, however, maintained that "not all FF (foreign fighters) are terrorists, and not all terrorists are FF.

"Thus, not all returnees systematically present a danger to the societies to which they return," it added.

Source: The International Centre for Counter-Terrorism

The researchers said that it was hard to understand the motivations of the returnees, but a previous study published by Dutch Security and Intelligence Service in 2014 offered various reasons for returning.

These included: "being disillusioned, being traumatised, (feelings of) betrayal, realisation of the atrocities, and regret, as well as having plans to recruit others or commit attacks in their countries of departure".

About 17 percent of them were female and 23 percent were converts to Islam, according to the latest study, published on Friday.

Most came from urban areas or peripheral suburbs of the continent's cities.

Source: The International Centre for Counter-Terrorism

Belgium - home to the attackers linked to last year's Paris shootings as well as last month's Brussels bombings - sent 41 fighters per million population.

Not only did Belgium contribute the most fighters compared to its population, but only 18 percent of them had returned, compared with 50 percent of those who had left from Denmark, the researchers said. Austria and Sweden followed in per capita terms.

In absolute terms, France was the largest source country for fighters who had left to fight alongside the Islamic State of Iraq and the Levant. The study counted more than 900 of them.

Germany and Britain also contributed large numbers.

Source: The International Centre for Counter-Terrorism

Source: Al Jazeera and agencies