CAIRO, Egypt – Foreign fighters returning home to the U.S., Britain and other countries after honing their skills in Syria's civil war are posing a "nightmare for security services," experts and officials say.
About 11,000 people have crossed into Syria seeking to help topple President Bashar Assad's regime – including about 60 who are believed to have traveled from America. However, many have now been left disillusioned by bitter infighting between rival rebel groups and some have given up their weapons and returned to the West.
Homeland Security Secretary Jeh Johnson earlier this month said that the U.S. government was "very focused" on the issue of militants returning to the country.
"Based on our work and the work of our international partners, we know individuals from the U.S., Canada and Europe are traveling to Syria to fight in the conflict," he added.
Raffaello Pantucci, a senior research fellow at London's Royal United Services Institute think tank, said recent history shows that "battlefields that have Sunni jihadist ideology have produced some sort of a threat."
The bombers who killed dozens of passengers on London's transport network on July 7, 2005 were "British nationals who went out to Pakistan and Afghanistan, before returning home to make the impact," he added.
Bilal Abdullah, one of two terrorists behind a failed 2007 attack on Scotland’s Glasgow International Airport, also trained with the group now known as the Islamic State of Iraq and the Levant (ISIL) before returning home to the U.K., according to Pantucci.
Bolstered by an influx of foreign fighters, ISIL has become one of the strongest actors on Syrian battlefields, Pantucci said. They are also active in Iraq where they gained control of Fallujah and Ramadi last month.
Due to their extreme ideology, it was thought that ISIL was linked with al Qaeda but the global terror group’s general command denied that earlier this month.
According to The International Center for the Study of Radicalization think tank, an estimated 11,000 foreign militants have entered Syria between late 2011 and December 2013 – including about 60 Americans.
They included former U.S.soldier Eric Harroun, a Muslim convert who fought with the Free Syrian Army before returning to America last March.
The FBI originally filed numerous charges against him, including using a weapon of mass destruction outside of the United States, which carries the penalty of either death or life imprisonment if convicted.
Harroun allegedly told the FBI that during his fighting in Syria he shot about 10 people but did not know whether he killed any of them, according to an affidavit obtained by Reuters. He told investigators that he hated al Qaeda and did not know any al Qaeda members, the affidavit said.
However, Harroun eventually pleaded guilty to an obscure law regulating munition exports and an Arizona Federal Court sentenced him to time served. He was released on September 19.
"It's not true to say that all of those returning from Syria will come back and be a terrorist threat," Pantucci added. "Perhaps a couple of them will, which makes it a nightmare for security services to track."
Many Western fighters have flown to Turkey before crossing the border, Pantucci said.
However, the non-native militants targeting Assad tend to be from countries neighboring Syria.
“We have many Jordanians who are jihadists and go to Syria,” said Dr. Musa Shteiwi, a director at the Center for Strategic Studies in Jordan, where the border with Syria has become increasingly porous over the course of the three-year civil war.
According to Shteiwi, they belong to many different groups including the ISIL and Jabhat al-Nusra, al Qaeda's official Syrian wing which is led by Ayman Zawahri. The International Center for the Study of Radicalization estimates up to 2,000 Jordanians may have joined the fight.
If peace talks were to be successful, some fear returning fighters could threaten stability in Jordan, which has taken in almost 600,000 Syrian refugees, according to figures released by the state-run Petra News Agency this week.
Insurgents returning to Egypt are also posing problems for security forces there, according to Sameh Saif Al-Yazal, a retired general who is now a director at the country’s Institute for Strategic Studies.
“The (Egyptian) jihadists coming in from Syria are a threat to everybody because they are people who are highly trained for violence and killing,” he said, adding that it was unlikely that those behind recent missile attacks on helicopters and car bombings had been trained inside Egypt.
Last month, 20 people died and more than 100 were wounded in coordinated bombings and gun attacks on two police headquarters in Cairo as well as a subway station.
Pantucci warned that even if those coming back did not intend to carry out a terrorist act themselves, they could have a radicalizing effect on young men who would be drawn to them or impressed by their battlefield heroics.
“Deep intelligence is required to build a picture about them,” Pantucci said. “Trying to figure out what they are doing, who they are connecting with both at home and abroad. Of course that’s incredibly difficult, not to mention expensive. There are no easy solutions.”
Robert Windrem and Lubna Hussain of NBC News contributed to this report. Henry Austin reported from London.