Nearly half of the 64 Americans who fought with Daesh, also known as ISIL or ISIS, as well as other terrorist groups, are believed to have survived the conflict, raising concerns that a some of those militants could return home to wreak havoc on US soil, researchers at George Washington University said.
Researchers examined federal court records and interviewed law enforcement officials and family members of US nationals who joined ISIL and al-Qaeda affiliates in Syria.
“We know that foreign fighters acquire both the military skills and networks to commit more lethal attacks if they return home," said Seamus Hughes, the deputy director of George Washington University's Program on Extremism.
Most of those who left the US to join terror groups in Syria or Iraq were US citizens or lawful permanent residents and averaged 27 years of age, the study found.
Following the collapse of Daesh in the region, American and European counterterrorism officials have scrambled to identify citizens who could pose a threat upon returning home.
"The information we have on people who've joined the fight in Syria is gathered from a lot of different places in a lot of different ways and guaranteeing accuracy is very hard,” a US counterterrorism official told ABC News. “It becomes a very complicated endeavor.”
US and European intelligence officials have recently stated that an unknown number of extremists from western nations, including the US, remain unidentified by authorities and remains a significant part of the threat.
"Law enforcement and counterterrorism officials remain concerned that there may be Americans, whose identities are currently unknown, who were able to avoid detection and who are now highly trained and experienced terrorists," said John Cohen, a former counterterrorism coordinator at the US Department of Homeland Security.
In 2016, former FBI director James Comey said the eventual return of foreign militants from Syria and Iraq would be challenging for western governments to track.
Approximately 350 of the more than known 800 extremists who traveled to Syria from Britain are believed to have returned to the UK since 2012, further complicating security concerns in a country that endured four terrorist attacks in 2017 after a dozen years without any incidents.