Domestic terrorism or homegrown terrorism is a form of terrorism in which victims "within a country are targeted by a perpetrator with the same citizenship" as the victims. There are many definitions of terrorism, and none of them are universally accepted. The United States Department of State defined terrorism in 2003 as "premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents, usually intended to influence an audience."
While there are many potential definitions of domestic terrorism, it is largely defined as terrorism in which the perpetrator targets his/her own country. Enders defines domestic terrorism as "homegrown in which the venue, target, and perpetrators are all from the same country." The term “homegrown terrorism” stems from jihadi terrorism against Westerners. Wilner and Dobouloz described homegrown terrorism as "autonomously organized radicalized Westerners with little direct assistance from transnational networks, usually organized within the home or host country, and targets fellow nationals."  The Congressional Research Service report, American Jihadist Terrorism: Combatting a Complex Threat, describes homegrown terrorism as a “terrorist activity or plots perpetuated within the United States or abroad by American citizens, permanent legal residents, or visitors radicalized largely within the United States.”
Under the 2001 USA Patriot Act, domestic terrorism is defined as "activities that (A) involve acts dangerous to human life that are a violation of the criminal laws of the U.S. or of any state; (B) appear to be intended (i) to intimidate or coerce a civilian population; (ii) to influence the policy of a government by intimidation or coercion; or (iii) to affect the conduct of a government by mass destruction, assassination, or kidnapping; and (C) occur primarily within the territorial jurisdiction of the U.S." This definition is made for the purposes of authorizing law enforcement investigations. While international terrorism ("acts of terrorism transcending national boundaries") is a defined crime in federal law, no federal criminal offense exists which is referred to as "domestic terrorism". Acts of domestic terrorism are charged under specific laws, such as killing federal agents or "attempting to use explosives to destroy a building in interstate commerce".
Homegrown terrorism is not new to the world. Security analysts have argued that after the end of the Cold War, military conflicts have increasingly involved violent non-state actors carrying out asymmetric warfare, of which terror attacks are one part. The United States has uncovered a number of alleged terrorist plots that have been successfully suppressed through domestic intelligence and law enforcement. The United States has begun to account for the threat of homegrown terrorism, as shown by increased volume of literature on the subject in recent years and increased number of terrorist websites since Abu Musab al-Zarqawi, the leader of al-Qaeda in Iraq, began posting beheading videos in 2003. A July 2009 document by the FBI estimated that there were roughly 15,000 websites and web forums that support terrorist activities, with around 10,000 of them actively maintained. 80% of these sites are on U.S.-based servers.
According to the Congressional Research Service’s study, American Jihadist Terrorism: Combatting a Complex Threat, between May 2009 and November 2010, law enforcement made arrests related to 22 homegrown jihadist-inspired terror plots by American citizens or legal residents of the U.S. This is a significant increase over the 21 plots caught in the seven interim years after the September 11 attacks. During these seven years, two plots resulted in attacks, compared to the two attacks between May 2009 and November 2010, which resulted in 14 deaths. This spike post-May 2009 shows that some Americans are susceptible to ideologies that support a violent form of jihad.
Roughly one-quarter of these plots have been linked to major international terrorist groups but an increasing number of Americans are holding high-level operational roles in these terrorist groups, especially al-Qaeda and its affiliated groups. The former CIA Director Michael Hayden called homegrown terrorism the more serious threat faced by American citizens today. The UK, likewise, considers homegrown terrorism to be a considerable threat. On June 6, 2011, Prime Minister David Cameron announced a wide-ranging strategy to prevent British citizens from being radicalized into becoming terrorists while at university. The strategy is intended to prevent extremist speakers or groups from coming to universities.
On July 23, 2019, Christopher A. Wray, the head of the FBI, said at a Senate Judiciary Committee hearing that the agency had made around 100 domestic terrorism arrests since October 1, 2018, and that the majority of them were connected in some way with white supremacy. Wray said that the Bureau was "aggressively pursuing [domestic terrorism] using both counterterrorism resources and criminal investigative resources and partnering closely with our state and local partners," but said that it was focused on the violence itself and not on its ideological basis. A similar number of arrests had been made for instances of international terrorism. In the past, Wray has said that white supremacy was a significant and "pervasive" threat to the U.S.
Domestic terrorism is often linked to lone wolf terrorism. Sociologist Ramón Spaaij defines lone wolf terrorism as an act of terrorism committed by one person who "acts on his or her own without orders from—or even connections to an organization". From the late 20th to the early 21st centuries, lone wolf terrorism in the United States has primarily been associated with white supremacy, Islamic fundamentalism, anti-social, and anti-government extremists such as Charles Whitman, Timothy McVeigh, Dylann Roof, Robert Bowers, Wade Michael Page, Ted Kaczynski, Eric Rudolph, Eric Harris, Dylan Klebold, James Holmes, Omar Mateen,and Tamerlan and Dzhokhar Tsarnaev. Many lone wolves share a common trait in that they seek acceptance from other groups but are typically met with rejection.
In their 2007 book Hunting the American Terrorist former FBI Deputy Assistant Director Terry Turchie and former FBI special agent Kathleen Puckett described six criteria to define a lone wolf:
There is no one path toward violence. Homegrown terrorists have been high school dropouts, college graduates, members of the military, and cover the range of financial situations. Recent[when?] research by Matt Qvortrup in the British Journal of Politics and International Relations has suggested that domestic terrorism is a result of lack of opportunities for meaningful political engagement. Some domestic terrorists studied overseas and were exposed to radical Islamist thought, while others took their inspiration from the internet.
Marc Sageman writes in his book, Leaderless Jihad: Terror Networks in the Twenty-First Century that, contrary to popular belief, radicalization into terrorism is not the product of poverty, various forms of brainwashing, youth, ignorance, lack of education, lack of employment, lack of social responsibility, criminality, or mental illness. He says that intermediaries and English-speaking imams, such as the late Yemeni-American cleric Anwar al-Awlaki (d. 2011), who are often found through the internet on forums, provide key roles in the radicalization process. Social networks provided in forums support and build upon an individual’s radical beliefs. Prison systems are also a concern as a place of radicalization and jihadist recruiting; nearly three dozen ex-convicts who attended training camps in Yemen were believed to have been radicalized in prison. The only constant appears to be "a newfound hatred for their native or adopted country, a degree of dangerous malleability, and a religious fervor justifying or legitimizing violence that impels these very impressionable and perhaps easily influenced individuals toward potentially lethal acts of violence," according to Peter Bergen and Bruce Hoffman's September 2010 paper for the Bipartisan Policy Center.
Training for potential homegrown terrorists is often very fast paced, or rushed, as some groups under attack by U.S. forces may feel the need to implement operations “more precipitously than they might otherwise occur,” according to Bruce Hoffman. This was the case with the failed Times Square plot carried out by Faisal Shazad. Pakistani Taliban (TPP) was on record as providing financing and four months of training for Shazad directly prior to his actions in Times Square. Shazad reportedly received only three to five days of training in bomb-making.
Some individuals go abroad to a region containing extremism, predominantly Pakistan, but also Iraq, Afghanistan,Yemen or Somalia. In the case of the London Underground bombers, Mohammad Sidique Khan, the operational leader of the cell, received military and explosives training at a camp in Malakand, Pakistan in July 2003. Later he took Shezad Tanweer to Karachi, Pakistan, in late 2004 to February 2005 where they crossed the border to receive training at al-Qaeda camps in Afghanistan.
Training and usage of recruits is varied. Some, such as Shahzad, received little training and ultimately failed in their goals. Others, like the sleeper agent David Headley’s reconnaissance efforts, were essential towards Lashkar-e-Toiba’s (LeT) success in the November 2008 Mumbai attacks.
Scholars say that some lone wolves may achieve objectives, but the vast majority of individual operators fail to execute their plans because of lack of training and planning. There is also a question as to whether such individuals are radical, or suffering other problems. The American convert, Abdulhakim Muhammad (née Carlos Bledsoe), who killed a U.S. military recruiter in Little Rock, Arkansas, and wounded another, had many other targets and plans, which went awry. It was not until some time after his arrest that he first claimed to have been working for Al-Qaeda in the Arabian Peninsula (AQAP). But, investigators found no evidence of this. The lead county prosecutor said that, aside from Muhammad's self-serving statements, it was "just an awful killing", like others he had seen. Bledsoe's father described his son as "unable to process reality." He was charged with capital murder and related charges, not terrorism, and pleaded guilty.
The American Nidal Hasan, the US Army major and psychiatrist charged in the 2009 Fort Hood shooting, had come to the attention of colleagues and superiors years before the shootings; they documented their concerns about his mental state. The Department of Defense has classified the event as "workplace violence" rather than terrorism, pending Hasan's court martial. Some observers believe that his personal characteristics are more like those of other mass murderers than terrorists; he did not belong to any group.
The Somalian Al-Shabab (“the youth”) have recruited strongly in Minneapolis and St. Paul, Minnesota. The 30+ Somali-Americans received training by senior al-Qaeda leaders in Somalia. Hoffman believes this indicates that radicalization and recruitment is not an isolated, lone-wolf phenomenon unique to Somali-Americans, but that there is terrorist recruitment infrastructure in the United States. After more than a dozen of 20 American recruits were killed in fighting in Somalia, the number of Americans going to join Al-Shabab has declined since 2007-2008.
“The Internet is a driver and enabler for the process of radicalization”, says a report of the Police Department of the City of New York of 2007. The internet has a wide appeal as it provides an anonymous way for like-minded, conflicted individuals to meet, form virtual relations, and discuss the radical and extremist ideology they encounter. The virtual network created in message boards or private forums further radicalizes and cements the jihadi-Salafi/racial supremacist message individuals have encountered as they build a community. The internet acts as an enabler, providing the aspiring jihadist/supremacist with a forum in which they may plan, share information on targets, weapons, and recruit others into their plans. Much of the resources needed to make weapons can be found on-line.
Inspire is an online English-language propaganda magazine published by al-Qaeda in the Arabian Peninsula (AQAP). Purported to be created by Samir Khan, a U.S. citizen and cyber-jihadist, the magazine uses American idioms and phrasing and does not appear to have British or South Asian influences in its language.
The magazine contains messages calling for western jihadists, like this one from AQAP leader Nasir al-Wahayshi, "to acquire weapons and learn methods of war. They are living in a place where they can cause great harm to the enemy and where they can support the Messenger of Allah... The means of harming them are many so seek assistance from Allah and do not be weak and you will find a way.”
STRATFOR suggests that the magazine is meant to "fan the flames of Jihad."
A non-exhaustive list of examples of U.S. attacks that have been referred to as domestic terrorism:
Perry and Hall exited the house, but were shot by officers who concluded they were trying to kill police rather than surrender.