WASHINGTON -- Before Abu Bakr al-Baghdadi became what many consider the world's most powerful terrorist, he was in U.S. custody.
U.S. forces released Baghdadi from an Iraqi prison in 2009 after four years in captivity. His reported parting words to American troops? "I'll see you guys in New York."
Baghdadi now leads a terrorist army of some 10,000 jihadists, known variously as ISIS or the Islamic State, that has conquered vast amounts of territory in Iraq and Syria.
And his repeated threats to attack America have not gone unnoticed.
"You will have a hard time finding any senior U.S. counterterrorism or intelligence official who won't say right now that the Islamic State, or ISIS, is the biggest threat that we're facing at the moment," said Matthew Levitt, director of the counterterrorism program at the Washington Institute.
Levitt, a former U.S. Treasury Department and FBI analyst, told CBN News the large influx of Western Muslims into ISIS's ranks -- from places like Britain, France, and Denmark -- poses a unique danger.
According to some estimates, up to 3,000 Western Muslims have traveled to Syria to join the jihad.
"They are recruiting a very large number and attracting, really, a very large number of foreigners, including Americans," Levitt told CBN News. "At least 100 or so have gone to fight, some with ISIS, some with Jabhat al Nusra in Syria."
"And also Europeans," he added, "and many of the Europeans that are being recruited are from European countries that have visa waiver programs with the United States."
Passport to Terror?
The fear is that these "holy warriors" will use their Western passports to return home and carry out attacks.
One French Muslim who had fought alongside ISIS did exactly that in May when he killed four Jews at a Brussels museum.
Moner Mohammad Abusalha, a U.S. citizen and Florida resident, took a different route. He blew himself up in a suicide attack against the Syrian military earlier this year.
Abusalha was not fighting for ISIS. But as the group expands its caliphate, or Islamic state, and becomes richer and more powerful, it becomes ever more attractive to Western recruits drawn to its success and social media savvy.
"What makes it such a threat to us is its intent and its ability to carry out attacks abroad, the fact that it has recruited and drawn into it so many foreigners, some of whom already have gone home," Levitt told CBN News.
"And some of their leaders, including al-Baghdadi, have articulated an intent to carry out attacks abroad, possibly even inside the United States," he said.
In one recent video, ISIS called America the "Protector of the Cross" and warned that the United States "shall soon be forced to be in direct confrontation by the permission of Allah, and the sons of Islam have been waiting for this day."
ISIS vs. Al Qaeda
Al-Baghdadi's claim to lead the world's Muslims has not set well with al Qaeda. Long considered the world's most notorious terror group, al Qaeda had a bitter falling out with ISIS earlier this year.
"I still think that al Qaeda is the bigger threat," Thomas Joscelyn, with the Foundation for Defense of Democracies, told CBN News.
"Because al Qaeda has a much more capable, deeper international network, the A-listers of the jihadi world have remained with al Qaeda. And that's a big cause for concern," he said.
Al Qaeda and its allies continue to expand their reach. The group now covers more territory than it did on 9/11, from Pakistan to Somalia, to Sinai, Libya, and beyond.
The al Qaeda branch that most worries U.S. intelligence officials is based in Yemen. It's been behind several plots against the U.S. homeland and has also recruited a number of Westerners.
"There are a small number of people in Yemen and Saudi Arabia who are making very ingenious bombs and very much have the intent to try to deploy them directed at the United States," Levitt said.
Iran and Hezbollah
Although al Qaeda and ISIS may now be rivals, there has been no such breach in the relationship between the world's leading state sponsor of terrorism and its most lethal proxy.
"Iran and Hezbollah have been involved in so many plots over the last few years that according to the director of National Counterterrorism Center, there have been times when the daily threats that are being briefed to the president are not all or primarily about al Qaeda or ISIS," Levitt explained. "The top one or two or sometimes even three have been Iranian or Hezbollah plots."
Levitt wrote a book called Hezbollah: The Global Footprint of Lebanon's Party of God , which details the group's international footprint.
In recent years, Hezbollah and Iran have attempted or carried out attacks against Jewish targets in Bulgaria, India, Thailand, and elsewhere.
U.S. authorities stopped a 2011 Iranian plot to blow up the Saudi and Israeli embassies in Washington, meaning Iran and Hezbollah have yet to strike on U.S. soil -- although they have killed American citizens abroad.
"It's very hard to say if and when they decide that they want to target U.S. interests," Levitt observed. "That they have the capability is very, very clear."
That capability would only increase if and when Iran acquires nuclear weapons -- an achievement that would embolden Iran's radical leaders and leave the world an even more dangerous place.