WASHINGTON - There have been 53 publicly known attempted terrorist attacks in the United States since 9/11, and of those, 43 have been categorized as homegrown plots, according to a government source.
Although there is little information publicly available indicating who was responsible for Monday's attack in Boston, the data show that most terrorist threats in the United States are domestic in origin.
Improvised explosive devices, so popular in places such as Iraq and Afghanistan, are often the weapon of choice, offering high impact with easily obtainable materials.
In the past six months alone, there have been 172 IED "incidents" in the U.S. - some 30 a month. Those range from actual explosions to the controlled detonation of devices that are found before they can go off.
Many are fireworks, pipe bombs, pranks, or other related activities, government sources say.
American IEDs often consist of a combination of homemade and commercially available explosives and can be triggered by cellphones. Monday's attack included an IED made out of a pressure cooker packed with explosives and shrapnel such as ball bearings and nails.
A pressure cooker bomb recipe was first shared with the general public in "The Anarchist Cookbook," published in 1971 during the Vietnam War.
"If this turns out to be a terrorist attack, it will be the fifth most violent attack in terms of casualties," said Bill Braniff, executive director of the National Consortium for the Study of Terrorism and Responses to Terrorism at the University of Maryland.
Despite what some people think, he said, the attack was reasonably sophisticated, with multiple explosive devices timed to go off nearly simultaneously.
"There is more investment the more of these explosives you generate," he said. "So there is a level of patience and planning."
Adam B. Hall, a former Massachusetts State Police crime lab employee and current forensic scientist at Boston University, said the explosions Monday at the marathon suggested to him that they were not caused by a "low explosive" such as black powder, which is a form of gunpowder. The large amount of white smoke and the sound the IEDs made point to more powerful "high explosives," he said, which would indicate a well-organized operation.
Government officials believe that enhanced vigilance in the United States since 2001 has made it more difficult for terrorist networks that are otherwise large and sophisticated enough to have global reach to strike the American homeland. That leaves smaller groups or individuals to carry out attacks, and for them, IEDs are about the only way to go.
Last October, for example, Mohammad Rezwanul Ahsan Nafis was arrested in a plot to bomb the Manhattan office of the Federal Reserve Bank - an operation inspired by the memory of Osama bin Laden, who had been killed a year and a half earlier in Pakistan.
In September 2012, the FBI arrested 18-year-old American Adel Daoud in a plot to detonate a car bomb outside a bar in Chicago.
The four worst U.S. terror attacks are 9/11; the World Trade Center bombing in 1993, which caused 1,048 casualties; the Oklahoma City bombing in 1993, with 818 casualties; and an incident in Oregon in 1984 in which 751 people were sickened by widespread salmonella poisoning at the hands of the Rajneeshis group.