The September 11, 2001, attacks in New York and Washington were the most destructive ever on U.S. soil. But law-enforcement officials have also long struggled with a range of U.S.-based terrorist groups. Domestic extremists include hate groups motivated by ultra-conservative ideals that are often anti-Semitic and racially motivated; ecoterrorists who use violence to campaign for greater environmental responsibility; and socialist groups who oppose the World Trade Organization. While homegrown Muslim extremists have proven more lethal in Europe than in the United States, U.S. authorities continue to worry about the prospect of attacks by militant Muslims who are American citizens. Domestic extremists have a “longstanding [sic] trend” of committing terrorist attacks on U.S. soil, according to the Federal Bureau of Investigation’s 2002–2005 Terrorism Report (PDF), the bureau’s latest comprehensive report on such incidents.

Yes, according to U.S. law enforcement officials. The September 11 attacks—the biggest and deadliest terrorist plot ever executed in the United States—were carried out by foreigners, but the twenty-four terrorist incidents that occurred between 2002 and 2005 were carried out by domestic extremists, according to the FBI. The most notorious recent case of domestic terrorism was the April 1995 truck bombing of the Alfred P. Murrah Federal Building in Oklahoma City, which killed 168 people and injured more than 500.

No. It has existed for more than a century, dating back at least to the 1901 assassination of President William McKinley. Extremists across the political spectrum—including white supremacists, Puerto Rican separatists, abortion opponents, and environmentalists—have used a variety of terrorist tactics to pursue their goals. According to the FBI, both domestic and international terrorist groups have since the early 1990s adopted looser organizational structures similar to the al-Qaeda network, which allows groups to plan larger attacks without fear of infiltration by law-enforcement agencies.

Just as differing definitions of terrorism are offered by government agencies and other experts, the meaning of domestic terrorism is also hard to pin down. The FBI, the lead federal agency dealing with domestic terrorism, has defined it as “the unlawful use, or threatened use, of force or violence by a group or individual based and operating entirely within the United States or Puerto Rico without foreign direction committed against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives.” The USA Patriot Act, passed in the wake of the September 11 attacks, defines domestic terrorism as criminal acts that are “dangerous to human life” and seem to be meant to scare civilians or affect policy. Civil rights groups have expressed concern that this definition is overly broad.

Not all politically motivated violence qualifies as terrorism (for instance, the FBI and some terrorism experts did not regard the Unabomber, Theodore Kaczynski, who says his antimodern beliefs were behind a seventeen-year mail-bombing campaign, as a terrorist), nor do all groups that espouse extremist ideas turn to terrorist acts. Experts do not consider all political assassinations or hate crimes to be terrorist attacks.

The FBI classifies domestic terrorist threats mostly by political motive, dividing them into three main categories: left wing, right wing, and special interest. Religious sects have also been connected with terrorist incidents.

Another type of domestic threat cited by federal law-enforcement officials in the period after September 11 is the alleged presence of Islamic extremists in the United States, operating either as an arm of a foreign organization or a homegrown cell. A 2007 survey by the conservative Heritage Foundation looks at least nineteen “foiled” terrorist plots, all within U.S. borders. Experts say often the groups linked to such plots are not wholly domestic; groups like al-Qaeda are establishing smaller, localized cells that rely on people who have longtime residence within a country to organize grassroots attacks.

The Southern Poverty Law Center (SPLC), a civil rights law firm that tracks hate groups, says these groups are not always considered domestic terrorists since they may be purely ideological, with no potential for violence. Right-wing extremist groups that engage in violence usually fit the criteria of a hate group. Left-wing and environmental extremist groups are not hate groups, according to the SPLC, because they do not espouse rhetoric that targets specific groups that have a defining characteristic.

Terrorist activity by anticapitalist revolutionary groups is considered left-wing domestic terrorism, the FBI says. In the late nineteenth century, immigrants from Eastern Europe sympathetic to the international anarchist movement launched what historians consider the first wave of domestic terrorism in the United States. Anarchists bombed Chicago’s Haymarket in 1886 and tried to kill the steel tycoon Henry Clay Frick in 1892. In 1901, an anarchist sympathizer named Leon Czolgosz assassinated President William McKinley in Buffalo, New York.

Another wave of left-wing terrorist activity began in the 1960s. Far-left groups such as the Weather Underground, the Symbionese Liberation Army, and the Armed Forces for Puerto Rican National Liberation (FALN) used bombings and kidnappings to draw attention to their radical causes. By the mid-1980s, however, left-wing terrorism began to wane.

The FBI says that anarchist and socialist groups with an anticapitalist and antiimperialist stance have diminished over the last several years and pose less of a threat than in the past. Left-wing terrorism is “a latent but potential terrorist threat.” Left-wing extremists caused much of the damage at the 1999 World Trade Organization meeting in Seattle.

Puerto Rican separatists represent another strain of left-wing extremists but experts say their activity is scaled back. In its heyday, the FALN tried to kill President Harry S. Truman, stormed the House of Representatives, and set off bombs in New York City, but Puerto Rican extremists today tend to confine their activities to Puerto Rico.

Attacks committed by people who favor individual freedoms over governmental regulation are classified as right-wing domestic terrorism. Such extremists may be motivated by issues of race, such as the Ku Klux Klan, or other issues, such as opposition to abortion or immigration. According to the FBI, right-wing terrorists often take “racist and racial supremacy and embrace antigovernment, antiregulatory” platforms. Far-right movements often blend political rhetoric with racial undertones, despite recent attempts to reach a broader audience by eliminating racial language. Authorities do not categorize people with extreme right-wing political ideals as threats unless the group they are affiliated with demonstrates a real potential for violence.

Yes. Attacks by left-wing or special-interest groups were the most common until the 1990s, when right-wing terrorists began staging more attacks aimed at civilians. The FBI says that the Oklahoma City bombing was carried out by far-right extremists who feared increased UN involvement in domestic policies, opposed stricter gun-control laws, and were enraged by “several confrontations between members of right-wing groups and law enforcement officers at Waco, Texas, and Ruby Ridge, Idaho.” A right-wing extremist, Eric Robert Rudolph, was also responsible for the 1996 Olympics bombing in Atlanta that killed two and injured more than one hundred. The decline of right-wing terrorist attacks since 2001 could be attributed to extremists’ anger shifting toward foreign entities and away from the U.S. government, the Los Angeles Times reported in March 2008.

The FBI reports that eight of the fourteen terrorist acts prevented between 2002 and 2005 were planned by right-wing groups. The others ranged from an anarchist plan to bomb a Coast Guard station, a prison-gang attempt to attack military and Jewish targets around Los Angeles, and a few people who attempted, individually, to establish ties with al-Qaeda.

Special-interest terrorism is perpetrated by the “extreme fringes” of social movements, such as animal rights, environmental, and antinuclear groups. According to the FBI, one of the most visible movements in the last ten years is termed “ecoterrorism.” Groups like the Earth Liberation Front and the Animal Liberation Front target facilities and materials that are perceived to be harmful to the environment or animals. Extremists advocating for better treatment of the earth and animals were responsible for twenty-three of the twenty-four terrorist attacks during 2002 to 2005. Environmental extremists are suspected of the early March 2008 arson that destroyed several new, unoccupied luxury homes near Seattle.

Generally the special-interest groups are composed of small, autonomous cells that are difficult to infiltrate because of their security and secrecy. Common tactics like arson, vandalism, and animal theft avoid physically harming humans and are meant to cause economic harm to the victims, according to the FBI.

Although environmental extremists were responsible for nearly all the domestic terrorist attacks between 2002 and 2005, right-wing extremists are still considered the most dangerous to the United States, says the SPLC. Right-wing extremist attacks are planned to target people, and if successfully carried out, intend to kill many civilians. Ecoterrorist attacks, on the other hand, aim to sabotage the infrastructure of businesses and corporations that endanger the earth; the groups do not aim to kill massive amounts of people.

The FBI says right-wing extremists have the potential to carry out the most deadly domestic attacks since they have a tendency to amass weapons and explosives and have “a propensity for violence.” Increasingly, right-wing terrorism threats come from what the FBI calls the “lone wolf” terrorist in the FBI Strategic Plan 2004-2009. Such an individual is a fringe member of a formal extremist group but acts alone instead of carrying out a group-planned attack. Despite limited funding, solo attacks can be deadly and are difficult to detect.