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, which authorities say were 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 in Boston. Instead, U.S. officials and counterterrorism experts said that, while the bombings may lead to incremental changes in efforts to secure such events, they exposed the limits of the extraordinary defenses erected over the past 12 years.
The United States has spent billions of dollars on counterterrorism efforts during that span, an investment that has accomplished much of its aim. Overseas operations have pushed al-Qaida to the brink of collapse, and domestic steps have dramatically reduced the country's exposure to an attack of the scale and sophistication of Sept. 11.
But the Boston bombings highlighted a lingering vulnerability that officials consider impractical, if not impossible, to eliminate. It centers on small-scale plots carried out by individuals who are unlikely to surface on federal radar.
"There's just no way to secure many large public events, and the kind of intrusive steps we would have to take are ones that no one would be willing to endure," said Rep. Adam Schiff, D-Calif., a former federal prosecutor and member of the House Intelligence Committee. "We've always known the limits of what we could do in a free society, and this week we saw those limits in all their horror."
National security and civil rights analysts said the U.S. government's response to the Boston bombings will depend on details that emerge from the ongoing investigation, specifically whether the brothers accused of carrying out the attacks had direct connections to a foreign terrorist organization, were inspired by the ideology of radical Islam or had other motivations.
The two men, Dzhokhar Tsarnaev and Tamerlan Tsarnaev, were immigrants who had been in the United States for roughly a decade. They were part of a family with ties to Chechnya, a volatile region where Muslim separatists have been engaged in a bloody campaign against the Russian government for decades.
Benjamin Wittes, a senior fellow at the Brookings Institution who tracks counterterrorism policy and law, said the administration would have to move carefully if it sought to expand surveillance in public spaces or increase monitoring of Muslim communities.
It's a "scenario in which you are almost powerless in a policy matter," Wittes said. "You obviously have to begin thinking about additional security at marathons and other events. But just as school shootings are really hard to prevent … I really don't think there's much more to do from a policy aspect."