Experts disagree on whether or not the United States has already been penetrated by the Islamic State in Iraq and Syria (ISIS). CNN tells us that Bob Baer, a longtime CIA operative in the Middle East — whose sources are probably as good as you can get — says "I have been told with no uncertainty there are ISIS sleeper-cells in this country."

On the other hand, we learn in the same article, "two U.S. officials tell CNN they have no indications of ISIS cells inside America right now. Still they are very concerned that ISIS fighters with [W]estern passports could travel to the U.S. and launch attacks."

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So what's a concerned citizen to do?

The logic of the situation leads to Baer's conclusion. Our borders are wide open — so there's plenty of opportunity for evil people to enter the country — and the terrorists would be delighted to strike a blow at the American homeland.

Moreover, there have been terrorist cells in the United States for decades. In 1994, three members of an Abu Nidal cell in St. Louis were sentenced to jail time. The FBI had been tracking them for some time, and the information became available to prosecutors when another member of the cell was convicted for murdering his daughter. And when I was in the government a few years earlier, I was aware that the FBI knew of at least one additional terrorist cell, probably Hezbollah.

Do you think it likely that the terrorists stopped infiltrating the United States in the last 20 years? I don't, especially since recruitment nowadays doesn't require the physical presence of an enemy agent; a good deal of that activity is now done online.

Indeed, we know that Americans are being recruited for terror operations. Take the case of Abdirahman Muhumed of Minneapolis, who was recently killed in Syria while fighting for ISIS. He had a job at the airport that gave him every opportunity to strike a serious blow: He had security clearances and access to the airplanes (he was employed by "a subsidiary" of Delta Airlines) and the tarmac.

If Muhumed could get such freedom of movement at the Minneapolis-St. Paul International Airport, there are probably others, with similar passions, in similar positions.

Reports on Muhumed suggest that he was probably an Internet recruitment, which in turn suggests that others in his cohort were clicking along the same lines (Minneapolis is a notorious hotbed of radical Somali jihadists). Interestingly enough — and contrary to what most of us were inclined to imagine — many of the most successful terrorist online recruiters are women, as detailed in this fascinating account in New York magazine:

"'We have a long history of women's involvement online and as propagandists' in jihadi movements, says Mia Bloom, a professor at the University of Massachusetts, Lowell's Center for Security and Terrorism studies and author of a book on female suicide terrorists. Dozens of purported jihadis have cropped up online in recent years, including over a dozen women.

Given so much desire and opportunity, it would be surprising if there were no ISIS cell here. And elsewhere.

Ledeen, the author of more than 30 books, is the Freedom Scholar at the Foundation for Defense of Democracies. He was special adviser to former Secretary of State Alexander Haig and a consultant to the national security adviser during the Reagan administration.