In the decade since the 9/11 attacks, homegrown extremists have committed thousands of violent crimes, from sabotage and property destruction to hate crimes to shootings to terrorist plots and conspiracies.  Many extreme movements also routinely engage in other criminal activities, such as frauds and scams or the acquisition of illegal weapons.

One way to examine the problem is to look at the most extreme results:  killings that can be attributed to homegrown extremists.  The true number of such deaths will never be known, as extremist connections in criminal cases sometimes take years to emerge, if at all, but there have been at least 201 deaths in the United States since 9/11 that can reasonably be associated with domestic extremists. 

The breakdown (see chart) may be surprising:  an overwhelming number of deaths (84%) can be attributed to just one source:  white supremacists.  Domestic Muslim extremists come in a distant second at 7%, but 13 of the 14 deaths in this category came from a single incident:  the deadly 2009 shooting spree at Fort Hood by Nidal Malik Hasan.  Anti-government extremists are third with 6% of the total—however, their killings are more targeted, including more than a third of the murders of law enforcement officers killed by extremists during this period.  The total also takes into account a few isolated deaths associated with left-wing extremism or anarchism, anti-immigration extremism, and anti-abortion extremism.

Two related issues are worth pointing out.  First a number of the killings by white supremacists involved traditional criminal motives rather than ideological ones.Overall, only about 49% of the killings were clearly related to a perpetrator's ideology or to extremist group concerns (such as killing suspected informants).  Another 42% were non-ideological or purely criminal in nature, from murdering relatives to killing police officers while in the commission of a non-ideological crime.  In about 9% of the killings, the perpetrators seemed to have mixed motives or their motives were not easily identifiable.  White supremacists were by far the source most likely to engage in non-ideological killings, both because many white supremacists also engage in traditional criminal activity and because white supremacist movements tend to attract many people with basic violent tendencies.

Second, homegrown extremists tend to be most dangerous when working alone as so-called "lone wolves."  In general, extremists who acted in groups or cells when planning terrorist acts were caught before they could actually commit the acts.  In contrast, extremists working alone, choosing more realistic targets and more obtainable weapons (guns rather than rocket launchers, for example), were far more likely to succeed in causing injury or death.  This has held true for white supremacists, anti-government extremists and domestic Muslim extremists alike.  

Just as the United States will continue to confront a variety of threats from international terrorists in the coming years, so too will it face the threat of violence from a variety of homegrown extremists who are already in the country in great numbers.