There is also cause for concern in that this was obviously a suicide operation—not in the direct way of a bomber who kills all his victims and himself at the same time by blowing himself up, but in the way of someone who conducts a spree, holding the stage for as long as possible, before he is cut down in a blaze of what he believes is glory. Here, think Mumbai.

Until now, it has been widely accepted in law-enforcement circles that such an attack in the U.S. was less likely because of the difficulty that organizers would have in marshaling the spiritual support to keep the would-be suicide focused on the task. That analysis went out the window when the Tsarnaevs followed up the bombing of the marathon by murdering a police officer in his car—an act certain to precipitate the violent confrontation that followed.

It has been apparent that with al Qaeda unable to mount elaborate attacks like the one it carried out on 9/11, other Islamists have stepped in with smaller and less intricate crimes, but crimes that are nonetheless meant to send a terrorist message. These include Faisal Shahzad, who failed to detonate a device in Times Square in 2010, and would-be subway bomber Najibullah Zazi and his confederates.

Is this, as former CIA Director Michael Hayden put it, the new normal?