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Nine facts about terrorism in the United States since 9/11

It's the 12th anniversary of the Sept. 11, 2001, terrorist attacks. Both Washington D.C. and New York City are commemorating the occasion with ceremonies this morning.

And this seems like an apt time to go back through the history of terrorist attacks in the United States and see how things have changed since Sept. 11.

First, some terms: The FBI says that there "is no single, universally accepted, definition of terrorism," but the U.S. federal code defines it as “the unlawful use of force and violence 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 best data on terrorism can be found in this big December report from the National Consortium for the Study of Terrorism and Responses to Terrorism. (Note that their analysis only runs through 2011 — so the April 2013 Boston marathon bombings aren't included.) Here are some  of the key findings:

1) Terrorist attacks and attempted attacks in the United States have become less frequent since the 1970s — though Sept. 11, 2001 was a huge exception.

There were 2,608 total attacks and 226 fatal attacks in the United States between 1970 and 2011.

Two big caveats about this chart. First, it only shows the frequency of terrorist attacks and attempted attacks, not severity. The attacks on Sept. 11, 2001, in New York City, Arlington, and Pennsylvania are counted as just four events, even though 2,996 people died, far more fatalities than all the rest combined. So keep that in mind.

The report also takes a very comprehensive view of terrorism. It includes the Sept. 11 attacks and the Oklahoma City bombing. But it also includes the murder of abortion-clinic doctors. And the guard shot at the Holocaust Museum in Washington in 2009. And all the instances of the Earth Liberation Front setting fire to SUV dealerships or police stations. It also includes serious but unsuccessful attempts — like the May 2010 attempted vehicle bombing in Times Square.

If we just look at the decade between 2001 and 2011, we still see that the number of terrorist attacks has declined since Sept. 11, although the number of fatal attacks has ticked up of late. That includes a fatal shooting at a Knoxville church in 2008, the shooting at Fort Hood that killed 13 people and injured 30 in 2009, as well as the Boston marathon bombing that killed 3 and injured 264 others in 2013.

2) Most terrorist attacks in the United States have relatively few deaths and injuries, but there have been 29 "mass-casualty events" since the 1970s:

In the wake of the Boston marathon bombings in April, START released a report noting that the average terrorist attack in the United States has resulted in 3.3 casualties on average ("casualties" includes both deaths and injuries).

That's just an average, however. There have still been 29 terrorist attacks in the United States since 1970 known to have resulted in more than 10 casualties. That includes the attacks on Sept. 11, which killed 2,996 and injured more than 6,000. It also includes the Boston bombings, which killed 3 and injured 264.

3) Law enforcement officials appear to  be getting better at thwarting terrorist attacks — but they can't stop all of them.

Sometimes sheer luck plays a role, too: "The highest proportion of unsuccessful attacks occurred in 2011, when four out of nine recorded attacks were unsuccessful," the report says. "In three of these attacks bombs failed to detonate before they were discovered, and in the fourth unsuccessful attack, shots were fired at the White House by an individual who has since been charged with attempting to assassinate President Obama."

4) Just about every part of the United States has been hit by some form of terrorist attack since 1970.

Keep in mind, though, that this includes all attacks, lethal and non-lethal. Here's a breakdown of the states that have seen the most attacks and fatalities:

Notice that New York and Virginia dominate the list of fatalities — again, that's because of the Sept. 11 attacks. (Pennsylvania is also up there, because of Flight 93.) After that is Oklahoma, mainly because 168 people died in the 1995 Oklahoma City bombing.

Puerto Rico, which is counted here, ranks third in number of attacks in large part because of the Fuerzas Armadas de Liberación Nacional, a paramilitary separatist group that was responsible for more than 120 bombings in the 1970s and 1980s.

Florida ranks high on the list in number of attacks because of the Earth Liberation Front, which has been particularly active in the past decade. But there haven't been many fatalities from these attacks. It's worth noting that the Earth Liberation Front and the Animal Liberation Front have been two of the most active groups since 1970, with 161 attacks total (and 84 since 2001). But they've killed zero people over the years — they mainly focus on setting fire to facilities, like SUV dealerships.

5) Since the Oklahoma City bombing, a greater portion of terrorist attacks in the U.S. have been carried out by individuals rather than groups:

6) The types of organized groups that carry out terrorist attacks, meanwhile, have become extremely diverse.

Al-Qaeda dominates this list, and the two eco-terrorism groups have been particularly active, though they both seem to be declining. But aside from that, terrorist groups seem to come in all types.

"These organizations are quite diverse," the report notes. "The [Al Qaeda in the Arabian Peninsula] attack was Umar Farouk Abdulmutallab’s Detroit suicide bomb attempt on Northwest Airlines Flight 253. The TTP attack was Faisal Shazhad’s attempt to detonate a bomb in Times Square. Members of the Minutemen American Defense, an anti-immigration militia group targeted a Mexican-American family. The KKK assaulted someone, and the Justice Department sent razor blades in envelopes to those conducting experiments on animals."

(Note that the "Justice Department" referred to here is an animal-rights group, not the federal agency.)

7) Bombings have long been the tactic of choice for terrorists in the United States:

Bombings have dropped in popularity over the last decade — accounting for just 27 percent of the attacks since 2001. (Again, though, this reflects the fact that eco-terrorists have dominated the raw numbers in the 2000s.) And, notably, guns have never figured heavily in U.S. terrorist attacks.

8) North America suffers far, far fewer terrorist attacks than most other regions around the world.

That tiny blue sliver on the bottom represents North America. Even Western Europe (in red) gets hit by terrorism more frequently. And the vast majority of attacks worldwide take place in South Asia (green) and the Middle East and North Africa (pink).

9) Your odds of dying in a terrorist attack are still much lower than dying from just about anything else.

In the last five years, the odds of an American being killed in a terrorist attack have been about 1 in 20 million (that's including both domestic attacks and overseas attacks). As the chart above from the Economist shows, that's considerably smaller than the risk of dying from many other things, from post-surgery complications to ordinary gun violence to lightning.

That said, terrorist attacks obviously loom much larger in our collective consciousness — not least because they're designed to horrify. So, understandably, they get much more attention.

Note: The post above is an updated version of this post that ran back in April after the Boston Marathon bombings.

Further reading:

--The Global Terrorism Database is a useful searchable site that lets you search and track terrorism attacks around the world since 1970.