Nearly a week after the attacks in San Bernardino, the picture of the attackers and what drove them remains murky. The husband-and-wife team of Syed Rizwan Farook, an American citizen, and Tashfeen Malik, a permanent resident who FBI officials said pledged loyalty to the Islamic State on Facebook shortly before the shootings, were both killed fleeing the scene and are now the subject of an evolving investigation into the impetus for their radical act. At a news conference on Monday, David Bowdich, the assistant director of the FBI’s Los Angeles office, told reporters that the pair had been “radicalized” for “quite some time” but “the question for us is how and by whom and where were they radicalized? Maybe there’s not a ‘by whom.’ Remember, oftentimes it’s on the Internet. We just don’t know.”

The FBI has not found evidence so far of an internationally coordinated plot, which raises the specter of homegrown jihadist terrorism—a category that President Obama, in his address to the nation on Sunday, pointed to as part of a “new phase” in the terrorist threat, one that preoccupied policymakers for years before the rise of ISIS. For example, the 2009 shooting in Fort Hood, Texas—until last week the deadliest jihadist attack on American soil since 9/11—was perpetrated by the Army psychiatrist Nidal Malik Hasan, who had tried multiple times to communicate via the Internet with Anwar al-Awlaki, the American-born imam and al-Qaeda leader later killed by a U.S. drone strike in Yemen. But it’s not clear Awlaki knew who Hasan was until after the shooting, which he praised on his website. In other words, rather than being actively groomed by al-Qaeda, Hasan may have been “self-radicalized,” seeking out ideological inspiration himself as opposed to being a passive recipient of extremist propaganda.

In the wake of that attack, Dennis Blair, then the U.S. government’s director of national intelligence, told a Senate committee that “we have made the complex, multiple-team attacks very difficult to pull off.” But “identifying individual terrorists ... using simple attack methods is a new degree of difficulty.” Obama echoed this theme on Sunday, remarking that “as we’ve become better at preventing complex, multifaceted attacks like 9/11, terrorists turned to less complicated acts of violence like the mass shootings that are all too common in our society.” It’s hard to organize a conspiracy involving communication and coordination among multiple people without getting caught, particularly if some of those people are overseas. It’s almost impossible to get precursor chemicals for sophisticated explosives. But it’s easy to plot in your own home, with your spouse, brother, or no one; and it’s easy to get a gun.

These dynamics have held steady in the United States for years. The urgent question now is whether ISIS, and in particular its much-touted mastery of social media, has appreciably changed the homegrown threat through some kind of superior ability to radicalize followers wherever they are based in the world. (The group does exhort Western supporters to travel to its self-proclaimed caliphate, but according to a recent study of ISIS sympathizers in the West, no one returning from such a journey has yet committed violence in the United States.)

But the causal relationship between individual radicalization and consumption of jihadist propaganda on the Internet is not straightforward. The process by which a person grows radical involves a complex mix of variables; in many cases, an individual may become more extremist offline, for example through the influence of friends or relatives, and then find ISIS online by virtue of its being the most accessible jihadist community to join. Or these processes could happen simultaneously.

In Obama’s words, “the Internet erases the distance between countries,” and “we see growing efforts by terrorists to poison the minds of people like the Boston Marathon bombers and the San Bernardino killers.” Dzhokhar Tsarnaev, who has been sentenced to death for his role in the 2013 Boston Marathon bombing, had al-Qaeda material on his laptop, including a bomb-making recipe from the Yemen affiliate’s English-language magazine Inspire, but pledged allegiance to no particular group in his claim of responsibility for the attack. That bombing took place before ISIS grabbed global attention in the summer of 2014, and certainly the rise of that group offers a new banner for “homegrown” extremists to claim. But as Joshua Keating has pointed out, it’s not clear how much jihadist radicals based in the West care about the difference between al-Qaeda and ISIS.

In a 2011 paper reviewing 10 years’ worth of terrorism prosecutions in the United States since 9/11, the bulk of them involving U.S. citizens, Karen Greenberg and her colleagues at New York University’s Center on Law and Security found that the top group affiliations among those indicted were “No Known Affiliation” (231 defendants); al-Qaeda (102 defendants); Hezbollah (72 defendants); and al-Shabaab (37 defendants). The crimes of which they were accused ranged, she and her coauthors wrote, “from misuse of a passport to attempt to use a weapon of mass destruction,” with only a handful of serious plots among them and criminal-conspiracy or material-support cases constituting the majority of the charges. (Since these numbers count prosecutions, many of them stemming from FBI sting operations, they are a reflection less of trends in terrorist activity than of law-enforcement priorities.)

More recent research suggests that the ISIS brand is on the ascent among aspiring and active American jihadists, though the methodology behind the numbers differs from that used in the Greenberg study. Since March 2014, according to a new report from George Washington University’s Program on Extremism, 71 people have been “arrested, indicted, or convicted” for what the authors call “ISIS-related activities” in the United States. Again, these people are predominantly U.S. citizens. And once again, “the overwhelming majority of those charged (73%) were not involved in plotting terrorist attacks in the U.S. Most U.S.-based ISIS supporters were arrested for intent to do harm overseas or for providing material support—namely personnel and funds—to fighters in Syria and Iraq.”

Another pattern revealed by previous research may yet hold true in the ISIS era. Reviewing al-Qaeda’s Internet-recruitment efforts in a 2011 paper, Brian Michael Jenkins of the Rand Corporation wrote that “many of the terrorists identified in this paper began their journey on the Internet. However, al-Qaeda has not yet managed to inspire many of its online followers to action. In the United States, its virtual army, with a few exceptions, has remained virtual.”

So what has changed? The content of the message and the venue where it’s delivered, primarily.

“Self-radicalization is not new, and in fact represents the norm that we have dealt with,” Jenkins told me. “What is new is that the very effective use of social media by ISIL, ISIS, whatever you choose to call it, has enabled them to reach a larger audience, a younger audience,” due in part to the “content of the communications, the vehicle of the communications.” To wit: The crowd attracted to the kind of brutality porn for which ISIS is famous is a very particular, self-selecting one. And Twitter, where according to the GW report “American ISIS sympathizers are particularly active,” may reach a younger audience.

Still, Charlie Winter, a senior research associate at the Transcultural Conflict and Violence Initiative at Georgia State University, said radicalization does not occur “in a bubble”—passive consumption of propaganda is not enough to transform an ordinary person into a murderer. It’s not the case, he said, “that individuals can find themselves on inevitable trajectories toward extremism if they go to the right place on the Internet and start hanging out with the wrong crowd. It’s nowhere near as simple as that.”

Online and off, before ISIS and since, the process has a lot to do with relationships, and ISIS’s social-media savvy can obscure the continuing importance of real-world connections in the radicalization process. Europe, for example, is a far more fertile recruiting ground for ISIS than the U.S. is, in part due to the stronger presence of in-person extremist networks there. “I think certainly in terms of logistics, there’s a kind of different recruitment landscape ... in North America than in Europe,” Winter said, “but I think that regardless of whether they’re operating online or offline, these networks act in a similar way and they’re offering similar things,” such as a sense of belonging or commitment to a cause.

To the extent that incitement to terrorism is a numbers game—even if the rate of translating such efforts into attacks is small, the number of successful attacks goes up with the number of attempts to incite them—ISIS’s command of Twitter does allow it to spread a wide net. And it’s almost certainly easier today for someone who, like the San Bernardino killers, has been “radicalized for quite some time” to find supportive material and inspiration online.

“That’s kind of a basic marketing theory,” Jenkins said. “But although this sounds perverse in the wake of the tragedy in San Bernardino, neither al-Qaeda nor the Islamic State, in response to their continuing exhortations to followers to take action, neither one of them is selling a lot of cars. As a marketing campaign, the yield is very, very low.” But that’s the nature of terrorism: Low-yield efforts can have a very high impact.