The Soufan Center found that at least 5,600 citizens or residents from 33 countries travelled to support the terrorist group, which last week lost its stronghold of Raqqa.
It noted that “while it will be hard to assess the specific threat posed by foreign fighters and returnees, they will present a challenge to many countries for years to come.”
Salman Abedi, the Manchester bomber, grew up in the northwest city, but had travelled into the UK from Libya just days before the attack.
Mashudur Choudhury became first person in the UK to be convicted of terrorist offences related to Syria conflict after travelling from Portsmouth to the country to attend a terrorist training camp.
The report notes that the flow of fighters came to a “virtual standstill” as Isis began to lose its territory in both Syria and Iraq and states implemented better measures to prevent travel.
It also reports that while returning foreign fighters have not as yet added “significantly” to the threat of terrorism around the world, the number of attacks inspired or directed by the Islamic State continues to rise.
MOST POPULAR STORIES ON YAHOO UK
Fireman who had world’s most extensive face transplant reveals why he ‘didn’t fear death’
Tired driver had ‘mini-sleep’ before four-carriage train crashed into buffers at Kings Cross
Millionaire surgeon to rebuild six-bedroom mansion which collapsed as he tried to install super-basement
These are the eight different prototypes aiming to be Donald Trump’s Mexico border wall
Briton jailed for ‘touching man’s hip’ in Dubai bar is freed and has passport returned
It notes that “all returnees, whatever their reason for going home” will continue to pose some degree of risk.
It says that states have not learnt how to deal with returnees — particularly women and children, as they struggle to understand how best to reintegrate them.
“Proper mental health and social support mechanisms will be especially relevant in the case of children,” it adds.
This week, the minister for international development said the only way to deal with British Isis fighters is to kill them.
Rory Stewart said that, “in almost every case”, militants in Syria can expect to be killed because of their “extremely hateful doctrine”.
He said they can expect to be killed because of the “serious danger” they pose to the UK’s security. The government said his opinions were in line with the UK’s policy.