Ever since al-Qaida attacked the United States in 2001, U.S. authorities have worked to detect and prevent the next big terrorist strike.
But officials and counterterrorism experts say the Christmas airline plot and last November's shooting at Fort Hood, Texas, may have shown al-Qaida that smaller-scale attacks also can prove unsettling, without the complexity and risk of bigger attempts.
The Christmas Day attempt to bring down a Detroit-bound flight — allegedly by a young Nigerian man with explosives in his underwear — was not successful. The attempt, however, shook the government, set agencies against each other and led to months of political second-guessing.
Short of mass casualties, the attack produced the kind of reaction that al-Qaida desires.
Now it appears that the group, which has prided itself on its ideological purism, seems to be eyeing a more pragmatic and perhaps more dangerous shift in tactics. The emerging message appears to be that big successes are great, but sometimes simply trying can be just as good.
It's not clear what Osama bin Laden and his senior leaders are thinking and plotting. But U.S.-born al-Qaida spokesman Adam Gadahn made a public pitch for such smaller, single acts of jihad in a recent Internet video.
"Even apparently unsuccessful attacks on Western mass transportation systems can bring major cities to a halt, cost the enemy billions and send his corporations into bankruptcy," Gadahn said in the video.
Officials believe this message has been evolving for the past year. It's turned upside down the prevailing wisdom that the next attack must be bigger and bolder than the one on Sept. 11, 2001.
"It's pretty clear that while al-Qaida would still love to have home runs, they will take singles and doubles if they can get them," said Bruce Riedel, a senior fellow at the Brookings Institution Saban Center and a former CIA officer. "And that makes the job of counterterrorism much, much harder."
The partisan bickering that followed the Christmas bombing may have played into al-Qaida's hands.
Counterterrorism officials note that al-Qaida leaders monitor the U.S. closely and watched the reverberations of the attack. They saw the scramble to boost security, the members of Congress criticizing agencies for intelligence and screening failures, the political drumbeat against the Obama administration's national security efforts and the agency leaders who rushed to blame each other.
The shift is ideological as well as tactical. Before Gadahn's latest video message, al-Qaida leaders bin Laden and Ayman al-Zawahiri had not seemed to embrace the call for smaller, more singular insurgent operations.
"Big al-Qaida still has too much of an ego. They still want big, synchronized, high-visibility attacks," said Jarret Brachman, an expert on jihadist groups. "They haven't yet said, 'Let a thousand flowers bloom.'"
Al-Qaida's senior leaders have worried that unleashing scattered and untrained insurgents who could make mistakes could do more harm than good to the greater jihadi message.
Brachman pointed to the November 2005 hotel bombings in Amman, Jordan, when one of the bombers set off his suicide belt in a wedding reception at a hotel rather than the lobby — killing the groom's father and 16 other family members and in-laws.
Killing vast numbers of innocent civilians — including fellow Muslims — was one of the factors leading to the erosion of al-Qaida in Iraq, a mistake the group doesn't want to make again.
In larger, more elaborate plots there are often many people involved, and the chances are greater that one will make a mistake or that law enforcement authorities will get a tip or notice something is going on.
For example, Najibullah Zazi's plot to bomb the New York City subway system late last year unraveled after investigators got a tip and gathered information from an imam who was communicating with the 24-year-old Afghan immigrant, as well as others at the same mosque.
Zazi, who has said he was recruited by al-Qaida and received training in a camp in Pakistan on how to build a bomb, was arrested in Denver before he was able to make his planned drive back to New York to set the plot in motion.
In contrast, officials allege that Maj. Nidal Hasan acted alone at Fort Hood after exchanging e-mails with radical Muslim cleric Anwar al-Awlaki.
Officials have raised concerns that while U.S. authorities were aware that Hasan was communicating with al-Awlaki, that information wasn't passed along to the military. But the assault did not include other people and occurred on a base where he was allowed to be, so it would have been difficult to predict or prevent.
The call for more individual jihad is not a new idea to al-Qaida followers on Internet forums. One writer scolded those who condemned Umar Farouk Abdulmutallab, the alleged airline bomber, as a failure.
"From my prospective living in the United States brother Abdulmutallab succeeded. Maybe he didn't achieve his full objective but you do not necessarily need to achieve a grade of 100 percent to pass the class," the writer said in an early January posting on the Ansar al-Mujahideen discussion forum, which is pro-al-Qaida and is now closed to new members.
"What Abdulmutallab did was instill a fear in Americans. This is a very significant accomplishment. An increased fear of flying, for example, can cripple the airlines and cause economic problems."
Another poster answered: "What did he accomplish? How many billions do you think they will spend to boost security that won't work anyway? He humiliated the Americans, afterward Newark Airport was on lock down for 6 hours because someone walked the wrong way. Success comes in many ways."
Gadahn, in his video, took a broader view, telling followers: "Jihad is neither the personal property nor the exclusive responsibility of any single group, organization or individual. ... Instead it is the personal duty of every able-bodied Muslim on the face of this earth."
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