American jihadists in Syria remain terror threat to US: Report A new report provides the most complete public accounting of the cadre to date.

 -- Many of the Americans who traveled to Syria to fight alongside Islamist extremists remain a potential threat to the United States, according to a new report released by George Washington University's Program on Extremism.

Researchers combed federal court records and interviewed law enforcement officials and family members of U.S. persons who joined the Islamic State, also known as ISIS, or Jabhat al-Nusra, al-Qaeda's affiliate in Syria, and determined that nearly half of the 64 Americans who fought with those groups are believed to have survived the conflict, raising concerns that a some of those combat-tested militants could return home to wreak havoc on U.S. soil.

“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 program's deputy director, who co-authored the report.

"The Travelers: American Jihadists in Syria and Iraq" provides the most complete public accounting of the American cadre to date. Most of them were U.S. citizens or lawful permanent residents and averaged 27 years of age, the study found, while only two were settled in the U.S. as refugees prior to joining the fight in Syria or Iraq.

Following the collapse of the so-called caliphate under a withering three-year western military campaign, counterterrorism officials in the U.S. and especially Europe — where most of the foreign fighters in the ranks of ISIS came from — have scrambled to identify citizens who could pose a threat upon returning home.

In the absence of concrete evidence of criminal activity, intelligence gathered on returning fighters might not be enough to convict them of material support to terrorism in court or even provide enough "derogatory" information to place them on a no-fly list in their home countries, where surveilling them properly is difficult to do with limited resources, U.S. and European officials say.

"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 U.S. counterterrorism official told ABC News. “It becomes a very complicated endeavor.”

An unknown number of jihadists from western nations including the U.S. remain unidentified by authorities, which also remains a significant part of the threat, U.S. and European intelligence officials have recently told ABC News.

"The idea that we know everyone who’s fought in Syria — there’s not a lot of comfort in the community saying that,” the U.S. counterterrorism official said. “We wouldn’t say we have that absolute certainty. That is challenging to obtain.”

John Cohen, an ABC News contributor and former counterterrorism coordinator at the Department of Homeland Security who maintains close ties to law enforcement, said jihadi groups in Syria have made efforts to hide westerners they recruited.

"Law enforcement and counterterrorism officials remain concerned that there may be Americans, whose identities are currently unknown, who were able to avoid detection, make it these conflict zones, and who are now highly trained and experienced terrorists," Cohen said.

Some of those on the list of 64 Americans -- including a few names made public for the first time by the Program on Extremism -- only became known to the FBI after they were already in Syria, and in one case, only after conducting a suicide bombing there for al-Qaeda.

"We identified a number of individuals whose names didn't hit against law enforcement until after they traveled to Syria for jihad,” Hughes said. “It's a hard problem to wrap your arms around.”

Former FBI Director James Comey in 2016 called the eventual exodus of foreign fighters from the caliphate in Syria and Iraq an "ISIS diaspora," which he said would be challenging for western governments to track.

Last year, the U.S. Joint Special Operations Command, working with agencies such as the CIA, FBI and foreign intelligence agencies, created a special "EXOPS" task force -- for "counter-ISIS external operations" -- to track foreign fighters returning home from the battlefield or traveling to other hotspots.

Approximately 350 of the more than known 800 fighters who traveled to Syria from the United Kingdom are believed to have returned home since 2012, further complicating security concerns in a country that endured four terrorist attacks in 2017 after a dozen years without any incidents.

Most of the British jihadi travelers have been deemed not a threat but officials have said the ongoing concern is that some could turn to violence at the encouragement of more than 3,000 "subjects of interest" who are the focus of 500 active counterterrorism investigations.

By comparison, the U.S., with five times the U.K.'s population, has approximately 1,000 open counterterrorism investigations, officials have said.

"Returning American ‘travelers’ will place a strain on law enforcement and intelligence resources as they assess who came back with nefarious intentions and who were disillusioned by their experiences in one of the most deadly terrorist organizations in the world," Hughes said.