Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University, said there were many examples of people wanting to fight for Al-Qaeda in their own country.

"Specifically, you've had a number of examples and cases where people who were trying to fight overseas have been turned back around to attack their homeland," he added.

The bombs used in Boston, pressure cookers filled with explosives, reflect the methods advocated by Inspire, the English language magazine published by Al-Qaeda in the Arabian Peninsula, the movement's Yemeni offshoot, which has also urged aspiring jihadists to launch attacks in their own countries.

Brian Jenkins, author of a Rand study on the profile of jihadists in the United States, said 74 percent of those involved in such plots were American citizens, of which 49 percent were born here and 29 percent were naturalized.

"Many of the jihadists identified in the cases discussed here began their journey toward radicalization on the Internet where they found resonance and reinforcement for their frustration and anger," he wrote.

Jihadists in America is a truly frightening thought.

How do we fight that? Eternal vigilance?