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Terrorism in the U.S.Definition: A common definition of terrorism is the systematic use or threatened use of violence in order to intimidate a population or government and thereby effect political, religious, or ideological change. This article serves as a list and compilation of acts of terrorism, attempts of terrorism, and other such items pertaining to terrorist activities within the domestic borders of the United States by non-state actors or spies acting in the interests of or persons acting without approval of state actors.

Here are a few basic things we know about the Boston Marathon blasts on Monday: Two bombs went off. At least three people died. Others lost limbs. More than 150 were rushed to the hospital. Police found at least two other suspicious packages in the area.

Beyond that, there's a lot we don't know — including who did this, or why.

“Any event with multiple explosive devices — as this appears to be — is clearly an act of terror, and will be approached as an act of terror," the White House told reporters on Monday. "However, we don’t yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic.”

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.” We still don't even know if the Boston blasts qualify — or if they were the work of a person with no goals except death.

One thing we can do, however, is provide some very general context about the history of terrorist attacks in the United States. A helpful set of basic facts and figures can be found in this big December report by the National Consortium for the Study of Terrorism and Responses to Terrorism. Here are some key findings:

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

There have been 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 September 11 in New York City, Arlington, and Pennsylvania are counted as just four events, even though there were 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 September 11 and the Oklahoma City bombing. But it also includes the murder of abortion-clinic doctors. And the guard shot at the Holocaust Museum 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 September 11, although the number of fatal attacks has ticked up of late. (That includes a fatal shooting at a Knoxville church in 2008, the assassination of abortion provider George Tiller in 2009, the shooting at Fort Hood that killed 13 people and injured 30 in 2009, and so on.)

2) 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."

3) 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 September 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.

4) Since the Oklahoma City bombing, a greater portion of terrorist attacks have been carried out by individuals:

5) 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.)

6) 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.

7) 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).

8) Your odds of dying in a terrorist attack are still far, far lower than dying from just about anything else.