He wasn't alone.
The American extremist killed fighting with Islamic militants in Syria is one of dozens of U.S.-born jihadists who have taken up arms with terror groups in Syria — posing a significant threat to national security, senior Obama administration officials told NBC News.
In response to NBC News’ exclusive report about American Douglas McAuthur McCain's being killed in Syria fighting for the Islamic State of Iraq and al-Sham (ISIS), the officials said dozens of other Americans have traveled to Syria and Iraq to help extremist groups, including — but not limited to — ISIS.
"The threat we are most concerned about to the homeland is that of fighters like this returning to the U.S. and committing acts of terrorism," one of the officials said after NBC News detailed how McCain, 33, of San Diego, was killed over the weekend by the Free Syrian Army, with his U.S. passport still in his pocket.
Jen Psaki, a spokeswoman for the State Department, also said Tuesday that ISIS and similar groups "have threatened to attack the homeland," adding, "We take those threats very seriously.”
ISIS has been able to attract disaffected foreigners as fighters because "they've been successful," mainly because the group hasn't faced major opposition, 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." "They have been able to take territory. Al-Qaeda doesn't take territory," Gelvin told NBC News.
But that very success could eventually drive away Americans and other foreigners, because once ISIS takes control in a region, it cracks down with a severe interpretation of Islam that quickly makes its leaders unpopular. "Once you get to know them, you get to hate them," Gelvin said. While 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.
— Peter Alexander and M. Alex Johnson