Adapting to the New Terrorism Landscape 25 Years After 9/11

In the 25 years after 9/11, the terrorism threat to the United States has changed considerably. The 9/11 attacks were planned over several years and carried out by 19 hijackers dispatched by a foreign-based organization with identifiable leadership and membership, a well-developed jihadist ideology, and defined political objectives. Most recent incidents of mass violence in the United States bear little resemblance to this model. Perpetrators typically are U.S. citizens with no organizational affiliation, their motives often blend elements of multiple ideologies, and they increasingly emerge from online communities bound less by political beliefs than by shared resentment and a fascination with violence. Their attacks kill far fewer people than 9/11, a real measure of counterterrorism success, but they emerge less predictably from a decentralized extremist ecosystem. The United States must better adapt to this new landscape to stop more attacks.

After 9/11, the U.S. government massively expanded its counterterrorism efforts to combat organizations such as al Qaeda. It created new institutions including the Department of Homeland Security and National Counterterrorism Center, expanded surveillance and watch-listing, and pursued terrorist leaders and operatives overseas. These measures made it far harder for a foreign organization to conduct an attack inside the United States. Today, lone actors dominate the terrorism landscape in large part because the counterterrorism apparatus stood up after 9/11 was successful. The emergence of the internet also likely accelerated this shift by giving individuals access to propaganda, tactical guidance, and new communities and audiences without requiring membership in a formal group.

Lacking formal training and support, lone actors are generally less capable and therefore less destructive than organized terrorist groups. At the same time, they are often harder to identify before an attack because they have fewer organizational connections that might expose their plans. The most lethal jihadist attack in nearly the last decade occurred on New Year’s Day in 2025, when Shamsud-Din Jabbar killed 14 people in a car ramming attack on Bourbon Street in New Orleans after pledging allegiance to the Islamic State. That Jabbar was radicalized in isolation and simply used his car to attack both deprived authorities of opportunities to discover his plans and made the attack far less destructive compared to an operation such as 9/11.

The growing prominence of lone actors has also complicated efforts to classify threats by ideology. Without a group doctrine, many perpetrators piece together ideas from multiple traditions. Take for example Mauricio Garcia. The 33-year-old Latino man spent years recording white supremacist and neo-Nazi beliefs, misogynist and incel grievances, personal humiliations, and admiration for other mass shooters before he killed eight people, seemingly indiscriminately, at a shopping mall in 2023. Or consider Patrick Crusius, who killed 23 people at an El Paso Walmart in 2019. In his manifesto, titled “The Inconvenient Truth,” Crusius combined white nationalist opposition to immigration with warnings about ecological collapse and denunciations of corporate power.

The problem extends to the edge of what counts as terrorism, where perpetrators mix political iconography with nonpolitical motives such as personal humiliation, resentment, a desire for notoriety, admiration for earlier killers, and self-awareness that their motive will be scrutinized. In August 2025, 23-year-old former student Robin Westman fired through the windows of a Minneapolis Catholic church during a school Mass, killing 2 children and wounding 27 people. Beforehand, Westman scheduled videos to appear online in which she called the plan “art” and displayed weapons covered with political memes, slurs, earlier killers’ names, and the phrase “there is no message.”

These attacks and others like them have produced an active debate among scholars over the remaining value that ideology provides for understanding lone actor terrorism. Some argue that new frameworks are needed as ideological boundaries dissolve and individuals combine ideas and move between causes. Others contend that these phenomena are not new and may only appear incoherent because analysts lack the categories or expertise to interpret them. These debates remain open. Nevertheless, lone actors share other patterns that offer opportunities to prevent violence regardless of their ideological beliefs.

Perpetrators of terrorism and other mass violence in the United States today are almost always American men acting alone or with one or two others. Many escalate to violence amid acute personal crisis, such as episodes of mental illness and periods of social isolation. Their paths are seldom invisible. Most display warning signs before an attack, confiding in family or friends, posting plans and grievances online, or unsettling the people around them in ways recognized in hindsight. And most must obtain a weapon, typically a firearm. These patterns can often be seen before an attack, and they mark the moments when families, communities, and authorities still have a chance to act. Prevention therefore requires clearer reporting channels, better behavioral threat assessments, and more accessible crisis services. A more ambitious strategy would reach even further upstream: improved access to mental health care and a greater emphasis on treatment for men, stronger support for families and students, safeguards that keep firearms from people at risk of committing political violence, and online mechanisms that make threats easier to report and content glorifying violence harder to spread.

Today, 25 years after 9/11, the United States should preserve the capabilities that have made another organized attack on that scale far harder to carry out. But those tools cannot by themselves address the more diffuse threat that remains. Preventing more attacks today requires greater attention to the behaviors and circumstances that offer opportunities to intervene.

Riley McCabe is an associate fellow with the Warfare, Irregular Threats, and Terrorism Program at the Center for Strategic and International Studies in Washington, D.C.