The most common country of origin for foreigners fighting for jihadist groups is Tunisia, followed by Saudi Arabia and Jordan. There are also at least 700 Britons fighting in Iraq and Syria as well, among 4,500 westerners overall.
The committee's report states:
These fighters pose a serious threat to the United States and its allies. Armed with combat experience and extremist connections, many of them are only a plane-flight away from our shores. Even if they do not return home to plot attacks, foreign fighters have taken the lead in recruiting a new generation of terrorists and are seeking to radicalise westerners online to spread terror back home.