WASHINGTON -- Aware that intensified American counterterrorism efforts have made an ambitious Sept. 11-style plot a long shot, al-Qaida propagandists for several years have called on their devotees in the United States to carry out smaller-scale solo attacks and provided the online education to teach them how.
"I strongly recommend all of the brothers and sisters coming from the West to consider attacking America in its own backyard," wrote Samir Khan, an American who joined al-Qaida's Yemen branch and emerged as a fervent advocate of homegrown, do-it-yourself terrorism before he was killed in an American drone strike in September 2011.
"The effect is much greater, it always embarrasses the enemy, and these types of individual decision-making attacks are nearly impossible for them to contain," Khan wrote in a Web publication.
The Boston Marathon bombing -- which the authorities believe was carried out according to instructions that Khan posted online -- offers an unsettling example of just how devastating such an attack can be, even when the death toll is low. It shows how plotters can construct powerful bombs without attracting official attention. It offers a case study in the complex mix of personality and ideology at work in extremist violence. And it raises a pressing question: Is there any way to detect such plotters before they can act?
The bombing killed three people, compared with 3,000 in the 2001 attacks. But it achieved the spectacular media impact that terrorists covet, marring an American institution with television footage of gruesome injuries and panicked crowds. Officials are worried about its copycat appeal.
The Boston case remains under investigation, and some facts set it apart from other domestic plots. FBI agents are still studying whether Tamerlan Tsarnaev, 26, who investigators believe carried out the attack with his younger brother, Dzhokhar, 19, received any training during a six-month visit last year to turbulent Dagestan in southern Russia. Intelligence agencies are reviewing whether two Russian warnings about the older brother in 2011 were handled properly.
At a news conference on Tuesday, President Barack Obama suggested that the bombers had acted on their own, saying that "one of the dangers that we now face are self-radicalized individuals who are already here in the United States." Obama said such plots "are in some ways more difficult to prevent."
So far, the Tsarnaev brothers appear to have been radicalized and instructed in explosives not at a training camp but at home on the Internet. Their bombs were concocted from inexpensive everyday items whose purchase set off no alarms: pressure cookers, nails and ball bearings, gunpowder from fireworks and remote controls for toys. Their choice of an open-air event meant no gate, metal detector or security inspection to pass through with their bombs.
In other words, as Dzhokhar told investigators, they followed the script from Inspire magazine, which Khan published in Yemen along with his mentor, the cleric Anwar al-Awlaki, who was killed in the same drone strike on Sept. 30, 2011. Al-Awlaki's incendiary sermons and Khan's training articles survived them on the Web, where the brothers found them.
Just a month before the Boston attack, the al-Qaida branch in Yemen posted on the Web the "Lone Mujahid Pocketbook," a compilation of all the do-it-yourself articles with jaunty English text, high-quality graphics and teen-friendly shorthand.
"R U dreamin' of wagin' jihadi attacks against kuffar?" the 64-page manual asks, using a pejorative term for unbeliever. "Have u been lookin' 4 a way to join the mujahideen in frontlines, but you haven't found any? Well, there's no need to travel abroad, because the frontline has come to you."
Some of the manual's ideas seem harebrained -- spilling oil on the road to cause car wrecks or welding blades to a pickup truck and driving into a crowd. But specialists say its bomb-making instructions are quite accurate. The Boston attack seems to have followed Inspire's tips: gunpowder emptied from fireworks, shrapnel glued inside the pressure cooker, a commercial remote control as detonator.
"The pressurized cooker should be placed in crowded areas and left to blow up," the manual says. "More than one of these could be planted to explode at the same time."
Philip Mudd, a former top CIA and FBI counterterrorism official, said the news from Boston came as no shock to those who reviewed the daily compilation of intelligence reports on terrorism. "Like everyone who looked at the threat matrix every day, I was surprised that this didn't happen sooner," he said.
He said he was struck by the lack of sophistication of the brothers, who made no attempt to hide or disguise their faces.
"They're angry kids with a veneer of ideology that's about skin-deep," Mudd said. He said the brothers might have as much in common with self-radicalized terrorists of completely different ideologies -- say, white supremacism or antigovernment extremism -- as with the committed al-Qaida operatives who organized the Sept. 11 attacks.
In the reports on Tamerlan Tsarnaev, Dr. Ronald Schouten, a Harvard psychiatrist who studies terrorism, sees what might be a classic portrait of a man vulnerable to extremist recruitment. He had failed at his dream of becoming an Olympic boxer and dropped out of college, disappointing his family and himself.
"People who fail," Schouten said, "sometimes latch onto a cause that makes their anger legitimate."
In recent years, al-Qaida propagandists have "made a particular effort to recruit lonely people who are looking for a cause," said Jerrold Post, a former CIA psychiatrist now at George Washington University and the author of "The Mind of the Terrorist."
He points to, among others, Maj. Nidal Hasan, the Army psychiatrist accused of shooting 13 people to death at Fort Hood, Texas, in 2009. Hasan was held up as an example for others in a two-part video released by al-Qaida's core group in Pakistan in June 2011 titled "You Are Only Responsible for Yourself," urging Muslims in the West to stage attacks without waiting for orders from abroad.
There is no consensus on how best to detect such homegrown attacks. Some law enforcement officials say that the Boston case vindicates their aggressive strategy of dispatching informants posing as al-Qaida operatives to meet young men who are flirting with violent jihad. Such sting operations often end when the aspiring terrorist attempts to detonate an ersatz bomb provided by the FBI.
But some Muslim activists say that identifying potentially violent people requires close, trusting relations between law enforcement and the Muslim community, which are undermined when informants invade the mosque and draw impressionable young men into talk of terrorism.
Had such trust prevailed in Boston, they say, perhaps Tamerlan Tsarnaev would have gotten more attention after two outbursts at a Boston mosque, where he denounced clerics' references to Thanksgiving and the Rev. Dr. Martin Luther King Jr. as contrary to Islam. The outburst, they say, might have led community leaders to go to the police.
John D. Cohen, a top counterterrorism adviser at the Department of Homeland Security, said the department was studying the common elements in the psychological profiles and behavior of people planning an attack -- whatever their ideology or motivation. Working with religious and community groups and local law enforcement, officials want to identify signs of impending trouble and find ways to intervene.
But Michael German, a former FBI agent who is now at the ACLU, said the problem with focusing on extremist views was that the vast majority of people who express them never turn to violence. Instead, the bureau should focus on illegal acts, he said.
In the 2011 Russian warning about Tamerlan Tsarnaev, German said, the key point was not that he had embraced radical Islam but that he planned to travel to Russia to join underground groups -- potentially an illegal act of support for a terrorist organization. But while a Customs official got word of Tsarnaev's plan to fly to Russia, no follow-up took place.
Finally, some specialists wonder whether it might have been possible to detect the brothers' bomb building. In 2011, Pfc. Naser Jason Abdo of the Army set out to build the pressure-cooker bombs described in Inspire magazine. He was arrested because of a blunder: he tried to buy explosive powder at the same gun shop near Fort Hood that Maj. Hasan had patronized in 2009. A clerk got suspicious.
The Tsarnaevs, by contrast, collected their powder from fireworks, including some bought at a Phantom Fireworks outlet in New Hampshire. William Weimer, Phantom's vice president, said the episode had prompted his company to seek training for sales personnel from the New York Police Department.
"Obviously the industry is abuzz about this," Weimer said. But he said there was nothing about Tsarnaev that flagged him as dangerous.
"He came in and asked, 'What's the most powerful thing you have?'" Weimer said. "That might sound suspicious. But 90 percent of the men, especially, who come in say, 'What's the loudest thing you sell? What's the most powerful?'"
By SCOTT SHANE
The New York Times