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Are Americans Like Douglas McAuthur McCain a Threat to America?

The news that an American died fighting with ISIS again raises concerns that Americans could return home to launch attacks on their own homeland.

The startling news that an American, Douglas McAuthur McCain, died fighting with extremist Islamist rebels in Syria again raises concerns that the Obama administration has sounded for months — that Americans could come back home hardened by conflict in Iraq and Syria and trained to launch terrorist attacks on their own homeland.

U.S. officials said in response to NBC News' exclusive report about McCain that "dozens" of Americans could be fighting at any one time with various extremist groups in the region — including the Islamic State of Iraq and al-Sham, or ISIS. With fear growing over the turmoil in the Middle East, U.S. intelligence and foreign policy agencies are racing to understand these radicalized Westerners and the potential threats they may pose abroad and at home.

Who's Joining the Fighters?

More than 10,000 foreign fighters are believed to be fighting with ISIS alone — nearly doubling the strength of the group, which is believed to have only 8,000 to 10,000 native fighters of its own, said James Gelvin, a professor of Middle East history at the University of California-Los Angeles and author of "The Arab Uprisings: What Everyone Needs to Know."

Opinions vary widely, but the broad consensus is that about 3,000 of those foreigners are from the West — most from Europe, particularly Britain, but also some from the U.S., Obama administration officials told NBC News. By and large, they're "people who are converts to Islam who have no prior experience to the religion [or] the culture, just like any other American," said NBC News terrorism analyst Evan Kohlmann of Flashpoint Global Partners, which tracks terrorist groups around the world.

The first wave of foreign fighters was attracted specifically to the battle to overthrow Syrian President Bashar Assad, Raffaello Pantucci, a senior research fellow for the London-based Royal United Services Institute, told NBC News. But at some point, that changed, and "the narrative right now is, 'I'm going to help build this Islamic State,'" Pantucci said last week.

They're people like Donald Ray Morgan, 44, of North Carolina, whom the FBI arrested on firearms charges this month. In June, he posted a photo to Twitter of him swearing an oath to the head of ISIS: "Mujahid pledging allegiance to Abu Bakr al-Baghdadi and Islamic State commanding good and forbidding evil."

And they're people like Shannon Maureen Conley, a 19-year-old Muslim convert from Arvada, Colorado, who pleaded guilty to providing assistance to a foreign terrorist organization. According to documents in U.S. District Court in Denver (PDF), Conley told an "active member" of ISIS that she was committed to "violent jihad against non-believers" and planned to travel to Syria to get special training to help ISIS.

Why Are They Going?

Because ISIS is "the biggest game in town," a senior U.S. intelligence official told NBC News.

"They've been successful," mainly because the group hasn't faced major opposition, Gelvin said Tuesday. "They have been able to take territory. ... These people are on the ground, and they're winning."

ISIS is known to have been specifically targeting Americans and other Westerners as far back as June, when it published a propaganda video featuring recruiters speaking fluent English.

"These are very young men, and it seems pretty cool when you see your buddies standing on tanks with Kalashnikovs. You want to be part of that," said Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalization at Kings College London.

What Do They Hope to Achieve?

"They want to expand the caliphate," Gelvin said Tuesday, although he doesn't believe that's really an achievable goal.

ISIS is led by a particularly hard-line group of believers called Takfiri, many of whom "consider it their right to decide who is a real Muslim," Gelvin said. That mindset is behind policies like medieval-tinged punishments for dissenters and the forced veiling of women that have made ISIS "extremely unpopular as they try to spread their own particular brand of Islam" in Syria, he said.

"The last caliphate lasted for 700 years," he said "This caliphate is not going to last all that long."

Are They a Threat in America?

The U.S. government and some Middle East experts certainly think so. As long ago as January, the FBI was devoting significant resources to the threat that Americans could go to Iraq or Syria, learn the latest terrorist techniques and return home to the U.S., Director James Comey said at the time.

"It's easy to get in and get out," Comey told reporters. "It's a challenge to identify people with bad intent and keep track of them, but we're spending an enormous amount of time on it."

"ISIS is a direct threat to the United States of America," Rep. Peter King, R-New York, said this month on NBC's "Meet the Press." "ISIS has hundreds of foreign fighters with them available to come to the United States to attack us. That's the reality."

On the same broadcast, NBC News counterterrorism analyst Michael Leiter, former director of the U.S. National Counterterrorism Center in the administrations of both President George W. Bush and President Barack Obama, agreed that it was reasonable to be concerned that "an American who goes to either Syria and Iraq, trains, gets experiences on weapons and comes back to the United States" could set off an improvised explosive device in a major U.S. city.

"That is a real and concrete threat," he said.

But while Gelvin, the Middle East expert at UCLA, said the threat of terrorist-trained Americans' returning home is a serious concern — "of course it is," he said — it's more likely that any Americans who survive their battles in Iraq and Syria will return home discouraged and disillusioned, not dangerous.

Speaking of ISIS leaders, he said: "Once you get to know them, you get to hate them. ... The idea of setting up sleeper cells in the United States that are going to be of any real significance is a fantasy."