By Peter Bergen
Editor's note: Peter Bergen is CNN's national security analyst, the author of "Manhunt: The Ten-Year Search for bin Laden - From 9/11 to Abbottabad" and a director at the New America Foundation. Jennifer Rowland is a program associate at the New America Foundation.
It's too early to tell who is responsible for Monday's bombings in Boston. Yet after an incident like this, everyone is looking to find out who did it and why.
One possible guide is recent history: In the years since the 9/11 attacks, dozens of extremists have plotted to use explosives to further their causes in the United States.
Of the 380 individuals indicted for acts of political violence or for conspiring to carry out such attacks in the U.S. since 9/11, 77 were able to obtain explosives or the components necessary to build a bomb, according to a count by the New America Foundation.
Of those, 48 were right-wing extremists, 23 were militants inspired by al Qaeda's ideology, five have been described as anarchists and one was an environmentalist terrorist.
But in the years since 9/11, actual terrorist bombings in the U.S., like the ones at the Boston Marathon, have been exceedingly rare.
Read Peter's full take on cnn.com/opinion.