With the news that a suburban Virginia man—a Pakistani-born naturalized U.S. citizen—has been arrested for surveilling stations of the Washington area's Metrorail system as preparation for bombing them, authorities have stressed that the public was never in danger. Of course it wasn't; the only apparent plot in this case was a phony one that was part of an FBI sting operation. The Bureau deserves applause for taking out of circulation one individual who, regardless of the artificiality of the plot that snared him, evidently was quite willing to assist an operation that could as easily have been a real plot to kill a lot of real people. The success of the sting operation is reassuring as far as it goes, but the case provides reminders that are less reassuring.

Notice who was stinging whom. It was not undercover agents posing as individual American would-be recruits to smoke out members of Al Qaeda or some other foreign terrorist groups. It instead was an individual American—who already had said and done some worrisome things to bring him to the FBI's attention—who mistakenly thought he was dealing with representatives of Al Qaeda. The incident thus fits into the pattern of several cases over the past couple of years, in which individuals in the United States took the initiative to seek out opportunities to commit terrorism. Contacts that they subsequently made with foreign extremists or groups were made from the individual's end. The man in the newest case appears to have not yet made, as far as we know, such contacts with real foreign extremists but evidently was on that same trajectory. Amid the heavy focus on foreign terrorist groups, and Al Qaeda in particular, in any public discussion of threats to the U.S. homeland, it behooves us to note where the initiation and instigation of possibly threatening operations is coming from.

The much-mentioned training that some of these individuals have gotten overseas once they have established contact with foreign groups is not the critical variable in determining terrorist threats to the U.S. homeland. As demonstrated in the case of Times Square bomber Faisal Shahzad, whose bomb-making training overseas evidently was not sufficient to keep him from looking for better information on the Internet and eventually putting together his failed contraption made of firecrackers, the training is not all that it often is cracked up to be. Moreover, little or no training is required for motivated people to kill a lot of other people in public spaces in the United States.

That gets to another reminder of the latest arrest, which is how inherently and unavoidably vulnerable our open and free society is to terrorist violence. One doesn't need ideas from an FBI sting operation to realize that a big city mass transit system is a juicy target—as has been demonstrated in London and Madrid. As the Washington Metro's police chief observed, “Transit is an open environment.” And the vulnerability of our public spaces goes far beyond mass transit. Shooting rampages in urban environments would be a cheap and easy way to make a large impact. We got a taste of that with the Fort Hood shootings, and the Washington area got a taste with the “D.C. sniper” case several years ago. It would take only a few people with modest resources, and not needing overseas training or other foreign support, to use similar methods in carrying out an operation that would be considered a major terrorist attack, on a scale somewhere between those earlier incidents and the attack in Mumbai in 2008.

There is a significant chance of such attacks occurring in the United States within the next few years. There is greater chance of attacks involving methods and targets such as those I just mentioned than of the kind of unconventional or spectacular operations that most capture our imagination.

The chance exists because there appear to be sufficiently motivated people inside the United States, motivated in large part by anger over certain U.S. actions. The likelihood of such attacks also stems from the impossibility of even the most creative and diligent efforts by law enforcement and intelligence agencies to find all the terrorist needles in a stack of other needles. And it stems from the ability of those motivated people to carry out their deeds with or without support from the foreign terrorists and terrorist groups we already know about.