Editor's Note: (Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of the new book "United States of Jihad: Investigating America's Homegrown Terrorists." )
(CNN) For many Americans, the big question after attacks of the magnitude we have seen in Brussels on Tuesday is: Could it happen here?
Certainly, the numbers of Islamic State in Iraq and Syria terrorist investigations in the United States today are quite sobering. According to U.S. Assistant Attorney General John Carlin, who is responsible for all federal terrorist investigations in the country, there were 60 cases prosecuted in the U.S. in 2015. That is more terrorist cases than in any year since September 11, 2001.
Fears of these potential threats have prompted a number of the presidential candidates to weigh in with their ideas on how to prevent such attacks in the United States. More on their "solutions" later, but they remind one of the great H. L. Mencken aphorism, "There is always an easy solution to every human problem -- neat, plausible, and wrong."
And the challenge of keeping the United States safe is a complex problem indeed. I interviewed Carlin at a major technology conference in San Francisco earlier this month, and asked him how many of these cases were precipitated by ISIS' adept use of social media. He said that almost all of them were.
ISIS, in other words, is radicalizing American citizens who are on their computers sitting in their bedrooms or downloading ISIS propaganda on their smartphones. And these Americans no longer have to travel to some overseas training camp to be indoctrinated by ISIS -- the group is also specifically catering to English-speaking militants and encouraging them to carry out attacks in countries such as the United States.
Early in 2015, ISIS released a booklet in English that was supposed to act as a guide on how to "be a secret agent" in a non-Muslim country.
The booklet offered militants tips about making bombs, hiding weapons, evading police surveillance and suggested recruits pick a Western sounding nickname so as to attract less attention. The group even suggested that recruits watch the Jason Bourne films for tips on employing evasion tactics.
In addition, ISIS publishes, in English, advice about how its followers can best communicate in encrypted fashion, telling recruits to use the Tor browser, which disguises the IP address of those who use it.
Unfortunately, ISIS' English-language propaganda is finding a number of takers in the United States. The FBI says that there are ISIS terrorist investigations in all 50 states, and that there are about 900 terrorism investigations in progress, the majority of which are ISIS-related.
In fact, over the past year and half, there have been five ISIS-inspired attacks in the United States. The most lethal was in San Bernardino, California, in December, when a married couple attacked office workers attending a holiday party killing 14. It was the most deadly terrorist attack in the States since 9/11. But there have been others.
In the fall of 2014, Zale Thompson allegedly attacked police officers with a hatchet in New York. He is believed to have been inspired by ISIS.
Last May, gunmen inspired by ISIS and also in direct contact with members of the terrorist group, opened fire at a cartoon contest of the Prophet Mohammed held in Garland, Texas. The gunmen, Elton Simpson and Nadir Soofi, were killed by police. And in November, a student at the University of California, Merced stabbed four people on campus, after visiting ISIS websites. In January, Edward Archer allegedly shot Philadelphia police officer Jesse Hartnett. Archer allegedly told police, "I pledge my allegiance to the Islamic State, and that's why I did what I did."
That's the bad news.
The good news, though, is that we are not so far seeing Americans trained by ISIS in Syria in paramilitary tactics then returning to the United States as we have seen with the multiple French and Belgian recruits to ISIS, including those who carried out the Paris attacks that killed 130 in November.
Last month, the U.S. director of National Intelligence noted that at least 6,900 militants from Western countries have traveled to Syria since 2012. But relatively few of these are Americans, and only seven* have been publicly identified as having returned to the United States.
Of these, one returned to Syria to carry out a suicide attack in 2014, and one has been charged with attempting an attack on the United States. Abdirahman Sheik Mohamud of Columbus, Ohio is believed to have left for Syria in mid-April 2014 and fought there before returning to the United States around two months later. The government alleges that before he was arrested he discussed some kind of plan (with an informant) to kill American soldiers at a military base in Texas. Mohamud has pleaded not guilty.
All this suggests that while ISIS-inspired attacks are certainly a strong possibility in the United States, they would likely be "lone wolf" attacks in which the perpetrator has no formal links to ISIS and has had no training from the terrorist group. The lack of ISIS training generally makes these lone wolfs far less lethal than the trained killers we saw in Paris and now in Brussels.
What U.S. counterterrorism officials are particularly concerned about is what they term a "blended" attack in the States, which is both inspired by ISIS but also directed by the terrorist organization. Under this scenario, an American recruit inspired by ISIS might reach out directly to members of ISIS in Syria over an encrypted social media platform, seeking some kind of specific directions for an attack.
We already saw a harbinger of this last May, when the FBI says one of the two ISIS-inspired militants who attacked the Prophet Mohammed cartoon contest in Texas sent more than 100 encrypted messages to a terrorist overseas, according to the FBI.
So, what can and should be done?
On Tuesday, Hillary Clinton called for more "surveillance." But the U.S. actually has quite a lot of surveillance already, and increasing surveillance is unlikely to have stopped the married couple from San Bernardino, who were not in touch with overseas conspirators and appeared to confine their attack planning to pillow talk of the type that can't be intercepted by the National Security Agency.
Also Tuesday, Republican presidential candidate Ted Cruz called for more police patrols in Muslim neighborhoods. Apart from the serious constitutional problems of targeting a particular religious persuasion in the United States, this would have the effect of alienating the very community that law enforcement depends on to provide useful information about who may be radicalizing in their midst.
And finally, Donald Trump, also running for the Republication nomination, advocated again for a temporary ban on Muslim immigration. Yet this would not work, either -- every single lethal jihadist terrorist attack in the United States since 9/11 has been carried out by an American citizen or a legal resident of the United States.
What is needed instead are more credible voices pointing out that ISIS is not creating an Islamist utopia here on Earth, as so much of its English language propaganda claims, but rather a living hell.
And who better to deliver that message in the United States than an American who deserted from ISIS for exactly this reason?
Muhammad Jamal Khweis, 26, of Alexandria, Virginia, was held by Kurdish fighters after allegedly deserting from ISIS earlier this month. Khweis gave an interview to a Kurdish TV station in which he said: "My message to the American people is: the life in Mosul [the Iraqi capital of ISIS] it's really, really bad. The people [that] were controlling Mosul don't represent the religion. Daesh, ISIS, ISIL, they don't represent the religion, I don't see them as good Muslims."
Should he return to the United States, U.S. prosecutors could throw the book at Khweis for joining ISIS, and he could get 20 years or more.
But they also could try something more creative -- a deal in which he tells prosecutors what he knows about ISIS in return for a reduced prison sentence. And one more thing: He would also have to appear before the American public explaining that ISIS is creating hell in the areas it controls.
*This number has been updated to reflect new data being released Thursday from New America.
Join us on Facebook.com/CNNOpinion.
Read CNNOpinion's Flipboard magazine.