The Sept. 11, 2001, attacks led to a massive buildup of security to make the country safe. Subsequent plots, including attempts to conceal bombs in shoes and underwear, prompted hasty additions to that edifice, as officials sought to fill in cracks that terrorists might exploit.

The bombings at the Boston Marathon, suspected of having been carried out by two young men who immigrated to this country about a decade ago, are likely to yield a more frustrating security postmortem.

So far, there have been no calls for a major addition to the nation's counterterrorism infrastructure, in part because it is difficult to identify a realistic measure that might have prevented the attacks.