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:

terrorist attacks since 1970

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.)

terrorist attacks 2001-2011

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

unsuccessful attacks

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:

terrorist map

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:

fatalities

Notice that New York and Virginia dominate the list of fatalities — again, that's because of the