And how could it?  There are untold thousands of people on such watchlists, and many of them present a more clear and present danger than the Tsarnaev brothers did until the day they carried out their crime.  It takes a minimum of five agents in five cars to conduct even a bare-bones surveillance of a single individual, and twice that many to follow someone who is surveillance-savvy.  There aren’t enough federal agents or police officers in the entire country to adequately monitor everyone about whom some suspicion has been raised.

But there are things that can be done. Chief among them is the monitoring of large public events by federal agents and local police officers trained in the art of identifying the criminal in the crowd.  And it is an art, but one based on science.  In every police department there are officers whose ability to spot a stolen car or an armed gang member seems so uncanny to their peers that it is assumed their success is the result of luck.  Similarly, at every international airport and port of entry there are customs agents whose “hit rates” in interdicting contraband far surpass those of their coworkers.  The successes these men and women have are not the result of luck, but rather of their ability to detect the subtle behavioral cues exhibited by people who drive stolen cars, carry weapons, or attempt to smuggle contraband into the country.

In viewing the photographs and video footage of the Tsarnaev brothers moving through the crowd before planting their bombs, I’m certain they displayed the same sort of behavioral cues that a trained observer would have detected.  For example, their backpacks appeared heavier than those carried by others in the crowd, something that might have attracted the notice of someone trained to spot would-be terrorists.  Also, at times they appeared to be together while at others they seemed to try to distance themselves from each other, a red flag to anyone looking to spot troublemakers.  And in some of the photographs they are looking in a direction different from everyone else nearby, an indication that they were interested in something other than the passing marathoners.  None of these cues, or even all of them taken together, would have justified a warrantless search of the brothers’ backpacks, but they might have attracted the attention of an observant police officer who then could have initiated questioning through a consensual encounter.  And a trained observer can almost always detect attempts at deception with just a few questions, usually within seconds of the first contact.

These techniques can be taught and learned, and they should be.  Vice President Biden may comfort himself by labeling the Tsarnaevs as “knock-off jihadis,” but even a knock-off can be every bit as deadly as the real thing.  Surely there are more Tsarnaevs out there waiting for their moment to strike.  Will it take another wake-up call before we figure out how to stop them?

(Thumbnail on PJM homepage based on a modified Shutterstock.com image.)