"A small handful" of Americans are believed to be fighting with ISIS in Iraq, U.S. officials tell NBC News.
U.S. intelligence, law enforcement and military officials have for months declined to discuss the possibility that Americans had joined the Islamic extremist group blamed for beheading 40-year-old New Hampshire journalist James Foley as well as atrocities against Iraqis. But they now acknowledge it is likely that one or more Americans - but no more than a "small handful" -- have joined the fray.
The officials, who spoke with NBC News on condition of anonymity on Wednesday, say they base that opinion on analysis and a string of recent arrests and investigations of U.S. citizens who have sought to join the group that is simultaneously fighting government forces in both Iraq and Syria.
"It stands to reason that one or some of those Americans who have gone to Syria may have linked up with ISIS. It would not be surprising," said one senior U.S. intelligence official. "In general, we have found that Western fighters, not just those from the U.S., are joining the biggest game in town and the biggest game in town is ISIS."
"If you marry that with the border disintegrating … with ISIS treating western Iraq and eastern Syria as one and … that we've seen Western fighters change allegiances to fight with the most successful groups … you have to assume an American has fought with them in Iraq.".
The official emphasized that the U.S. has no specific intelligence identifying any Americans as having fought with ISIS.
Other officials tell NBC News that a small number of Americans - about a dozen at any given time -- are believed to be embedded and fighting with ISIS in Syria, and that some may have crossed into Iraq.
U.S. intelligence and counter terrorism officials have previously estimated that up to 100 Americans have tried to join groups in Syria fighting to topple the regime of Bashar Assad, and that about 70 have actually made it.
"Once an American arrives in this no man's land, our ability to track them clearly diminishes."
The difficulty in estimating the number of Americans increases exponentially once they cross the Syrian or Iraqi borders, said one intelligence official.
"As an intelligence community, it is a challenge for us," the official said. "Once an American arrives in this no man's land, our ability to track them clearly diminishes. That's why it's best to stop them before they go because we lose insight on where they're fighting and with who."
In fact, FBI Director James Comey said last week that his agency cannot be certain of even the range of Americans who've traveled to Syria.
"When I give you the number of more than 100, I can't tell you with high confidence that's a 100 of 200, that's a 100 of 500, that's a 100 of a 1,000 or more, because it's so hard to track," Comey said in remarks at the Dallas FBI Field Office.
Evan Kohlmann, a partner with Flashpoint Intelligence and an NBC News terrorism analyst, points to three recent cases of Americans prosecuted for attempting to join the group.
- Adam "Fadi Fadi" Dandach, 20, who was arrested by FBI at Los Angeles International Airport in early July 2014 and later told agents that he was trying to travel to Syria to join ISIS.
- Donald Ray Morgan, 44, of North Carolina, who was arrested by the FBI for firearms violations on Aug. 2. Investigators found he had posted numerous pro-ISIS messages on social media and believe he was seeking to join ISIS. On June 29, he posted a photo to Twitter of him swearing an oath of loyalty to the head of ISIS: "Mujahid pledging allegiance to Abu Bakr al-Baghdadi and Islamic State commanding good and forbidding evil," it reads.
- Shannon Maureen Conley, 19, of Colorado pleaded guilty on April 8 to federal charges that she attempted to fly to Turkey on the first leg of a journey to join ISIS.
In addition, NBC News has learned of at least two federal grand jury investigations involving Americans who have gone to Syria. In one case, more than one American is under investigation in the United States after having returned from Syria; in the other, an American is being investigated as he or she remains in Syria, law enforcement officials tell NBC News.
Overall, U.S. and allied intelligence agencies put the number of Westerners who have traveled to Syria at 1,000, about half of whom are believed to be from the United Kingdom.
"The narrative may be changing," says Raffaello Pantucci, a senior research fellow for the U.K.-based Royal United Services Institute. "The narrative used to be quite clearly, 'I'm going to fight the Assad regime.' The narrative right now is 'I'm going to help build this Islamic State.'"