WASHINGTON â€" The shoe bomber, the underwear bomber, and the Times Square bomber each failed in their primary objective, to kill Americans, but they contributed to a mood of both political resolve and deep worry about the country's terror defenses in the decade after 9/11.
Following the Boston Marathon attack, political leaders gave voice once again to those sentiments, but the reaction was magnified by the shocking toll, with the deaths of three people and widespread injuries from flying shrapnel.
Some politicians wondered whether the absence of a successful terrorist strike on US soil since Sept. 11, 2001, has lulled Americans into believing that all of the security screenings, intrusive searches, and ubiquitous surveillance cameras are keeping them safe â€" that they don't have to worry any more about every unattended backpack on the sidewalk.
''The complacency that prevailed prior to September 11 has actually returned," Senate Minority Leader Mitch McConnell, the Kentucky Republican, said in a speech on the Senate floor Tuesday morning. "And so we are newly reminded that serious threats to our way of life remain."
McConnell's assertion â€" part of a debate that for the moment has diverted attention from such issues as immigration and gun control â€" poses anew questions about how Americans respond to the threat of terror.
Have they grown so desensitized to the "if you see something, say something" warnings that they are less likely to say anything? Polls show terrorism has certainly faded as a public concern in the last dozen years since the 9/11 attacks.
''That's just human nature. And that's good. We don't want people obsessing about being afraid -- especially afraid of something that has a very, very low likelihood," said David Schanzer, a professor at Duke University who specializes in terrorism, Guantanamo Bay, and national security policy.
Former senator Joe Lieberman said the explosions in Boston brought to life "one of the nightmares" he had as a chairman of the Senate Committee on Homeland Security and Governmental Affairs: an attack on "relatively undefended massive events where there's a lot of people."
''The pain and insecurity that was brought on by the attacks of 9/11 naturally over time have diminished among the general public," the Connecticut independent said in an interview. "But I think people are still aware that we're living in a post 9/11 world. 1/8Monday3/8, tragically, brings it all back. It brings back the pain and the insecurity and the anger and the resolve not to let terrorism change our way of life."
President Obama said Tuesday investigators do not know whether the bombs that killed three and wounded scores at the Boston Marathon were an act of a single person or a an organized group -- or why they were planted.
But he struck a resolute note as he sought to reassure the country that the full force of US power and the justice system will be used to hunt down those responsible.
''American people refused to be terrorized," Obama said.
Obama, who is scheduled to visit Boston on Thursday, cited what he described as acts of heroism at the Marathon finish line shortly after the bombs exploded, creating a chaotic and bloody scene. He lauded exhausted runners who "kept running to the nearest hospital to give blood" or tore off their own clothes to make tourniquets for the wounded.
Past attempted bombings have triggered more security measures at airports -- requiring that shoes be removed, and body scanners to ensure that passengers are not carrying explosives in their undergarments. The Marathon bombing reveals how, outside of airliners, Americans remain exposed to a determined attacker.
''Even though we're so much better on pure preparedness â€" light years â€" the situations posed and the threats posed are greater, and they're harder to detect," said Representative William R. Keating, a Democrat from Massachusetts and a member of the House Committee on Homeland Security. "I'm not saying it's the case here, but threats of lone wolves and people dealing in groups of two or three make it harder to detect. You just don't have the forewarning."
Senator Susan Collins, a Maine Republican and a member of the Senate Committee on Homeland Security, said in an interview: "I believe that a lot of Americans have grown more complacent about the threat and this is a horrific reminder that the threat of terrorism is so very real and that there are those who are determined to harm innocent Americans using horrific means,"
Some lawmakers denounced budget cuts for law enforcement and raised questions about immigration legislation.
''Some of the speculation that has come out is that yes, it was a foreign national and, speculating here, that it was potentially a person on a student visa. If that's the case, then we need to take a look at the big picture," Representative Steve King, a Republican from Iowa, told National Review Online.
''We need to take a look at the visa-waiver program and wonder what we're doing," he added. "If we can't background-check people that are coming from Saudi Arabia, how do we think we are going to background check the 11 million to 20 million people that are here from who knows where?"
Several others, including Senator Marco Rubio, a Florida Republican, warned against jumping to conclusions. And reports indicated that a student from Saudi Arabia, who reportedly was questioned, had nothing to do with the bombing.
''We should really be very cautious about using language that links these two things in any way," Rubio told reporters Tuesday. "We know very little about Boston other than that it was obviously an act of terror. We don't know who carried it out or why they carried it out, and I would caution everyone to be very careful about linking the two."