It's too soon for detailed discussion, but a couple of things struck me in listening to the news coverage of this outrage. First, this has the fingerprints of al Qaida all over it. There was an early report that five additional bombs were discovered, unexploded, for a total of seven bombs including the two that did. Classic al Qaida tactics are to set off one or more coordinated explosions and then secondary blasts a short time later to catch the emergency personnel responding. I believe this was the tactic employed in Boston, except that the second set of explosive devices did not explode.
Had all seven bombs detonated, the loss of life would have been easily in the hundreds and al Qaida, or one of its splinter groups, would have had the follow-up attack predicted by Osama bin Laden after 9/11. It's generally accepted that they have been desperately trying to accomplish another major terror attack in the U.S., unsuccessfully which is a tribute to the safeguards instituted after 9/11 and the effectiveness of Homeland Security. President Obama wisely has kept much of the Bush-era policies, like the USA Patriot Act and electronic monitoring (ELINT), in place.
I believe this was to have been the big one. The fact is this attack, bad as it was, was not as horrendous as it could have been had all seven bombs detonated. The reason they didn't appears to have been that cell phone towers were shut down immediately after the initial blasts, according to early news reports. Al Qaida frequently uses cell phones to set off IED's. The folks who set up this procedure and implemented it so promptly deserve our gratitude. They are the real reason al Qaida did not get the massive terror conflagation in the U.S. that they so desperately desire.
Rest in agony, Osama.
NOTE: News reports at this time are confused and contradictory. The newspaper today (Tuesday) reports only two additional bombs were found and that cell phone service was operational. Until the situation clarifies, please treat this post as purely speculative. I still believe al Qaida was involved.