When Mohamed Merah died in a brief, ferocious gunfight with a French SWAT team today after a 32-hour siege in Toulouse, his own killing spree came to a definitive end. But there is growing concern among American and European law-enforcement officials that this petty-thief-turned-jihadist-murderer represents dangerous changes in the way terrorists operate that are extremely hard to monitor or to stop.

Since news broke of Merah’s attack on a Jewish school in Toulouse on Monday, police in New York City and Washington, D.C., have stepped up protection of potential targets in their cities. Some of this is evident, with an increased presence by uniformed cops; some of it is not visible to the public. Dozens of sites, most of them identified with Jewish faith and culture, are being guarded. “You don’t want people to be overly concerned,” New York Police Commissioner Ray Kelly told The Daily Beast on Wednesday, “but you want people to know that we are taking care of business.”

That kind of protection, however, is the last line of defense. The key to stopping terrorist attacks before they start is intelligence collection, and the services in France, widely recognized as some of the most effective (and ruthless) in the world, managed to miss completely the threat posed by this 23-year-old Frenchman of Algerian descent.

It’s not as if they’d never heard of Mohamed Merah or his family. His older brother, Abdelkader, was known to be “implicated” at a low level in a network that smuggled jihadists to Iraq in 2007. The fact that Merah himself went to the Afghanistan-Pakistan region twice in the last two years apparently set off no alarm bells. In 2010 he was picked up by Afghan police at a checkpoint and turned over to American troops, who “put him on the first plane bound for France,” according to Paris Public Prosecutor François Molins. In 2011, after two months in the Afghanistan-Pakistan region, Merah returned to France again last October, ostensibly because he’d contracted hepatitis. Five months later he launched his murderous campaign in the Toulouse area, killing three French paratroopers, a rabbi, and three little Jewish children.

French authorities continue to play down the 23-year-old Merah’s claims before his death that he was tied to al Qaeda, but their explanation for skepticism is weirdly circular. He went to Afghanistan, said Molins, without using routes “known to the French and foreign specialized services” and without using “the facilitators who are targeted by [those services] and without passing through the countries that are usually under surveillance.” A security source close to the French presidency tells The Daily Beast that on the first trip Merah wasn’t on the ground long enough to draw attention, and on the second he traveled with a visa, supposedly in search of a bride. (Frenchmen of Algerian backgrounds looking for Afghan or Pakistani brides apparently raise few suspicions among the French intelligence services.)

“He knew how to avoid them,” says a senior American intelligence officer. “The resilience of the threat is just astonishing. The remarkable thing about terrorism is how it changes, how agile it is.” As far as this official is concerned Merah is “clearly a jihadist salafist, whether he is card-carrying al Qaeda doesn’t really matter.”

There once were fairly clear distinctions between the core al Qaeda organization, loose affiliates like the Yemen-based al Qaeda in the Arabian Peninsula, and so-called “lone wolf” terrorists acting on their own with inspiration and instruction garnered from jihadist Web sites. But the differences are harder and harder to discern. One component comes under attack, others take up the relay. As Commissioner Kelly puts it, “You see a weakening of the core but a strengthening of the affiliates.” And the lone wolves are not always so lonely.

Merah was following to the letter the public directives of the late Osama bin Laden and his successor Ayman al-Zawahiri. In a call to the France 24 international television network and in dialogue with police negotiators during the Toulouse siege Merah gave three reasons for his savagery: to avenge the killing of Palestinian children by Israel, to punish France for its ban on the burqa for Muslim women, and to exact a price from France for sending troops to Afghanistan. In October 2010, as France 24 reported at the time, those were the reasons that bin Laden gave for calling on jihadists to target the French specifically.

“This is right out of the bin Laden-Zawahiri playbook,” says Bruce Hoffman, director of the Center for Peace and Security Studies at Georgetown University. “And there are plenty of cases in past years where people at first dismissed al Qaeda involvement, and then it does turn out to be involved.”

When the core al Qaeda organization was stronger and bin Laden was alive, it focused all its attention on trying to recreate the spectacular impact of a 9/11. That operation took hundreds of thousands of dollars and extensive logistical support. But the relentless drone attacks on al Qaeda’s leadership and the daring SEAL raid that killed bin Laden last year have undermined al Qaeda core’s ability to plan or execute such apocalyptic spectaculars.

Terrorist affiliates like the one in Yemen, advised by American-citizen Anwar al-Awlaki, lowered the bar considerably. They were linked to several failed bomb plots and to shootings by lone gunmen in the United States who targeted soldiers in Texas and Arkansas. The United States later managed to kill Awlaki, but with or without him, a shooter like Merah found it easy to adopt similar tactics: he chose to target soldiers who weren’t on duty around a city that was not perceived as a likely terrorist target, then he targeted Jews and their children at a school. Such plots are relatively easy to plan, easy to carry out, difficult to detect, hard to stop—and still get global publicity.

“The targets are highly significant; they seem to have been very seriously thought out for symbolic value,” says Hoffman. “Even if [Merah] isn’t directly part of the al Qaeda firmament it may not matter because, for Zawahiri looking at the news, he’s thinking, ‘They are listening to me vicariously, and this strategy works.’ ”

For all these reasons, the danger posed by this improvisational terrorism, even if it is carried out by only a very few individuals, can have a major impact on public confidence. “We see no reason to think the threat has diminished,” says Kelly. Yet this is also coming at a time when the public is increasingly skeptical about the measures law-enforcement agencies have taken over the last 10 years to penetrate and disrupt terrorist operations and mount lines of defense if some slip through.

“There’s a backlash against the ‘war on terror,’” says Hoffman. Whether it comes to draconian airport security or what’s been portrayed as pervasive surveillance by law enforcement, “people just want to forget all this stuff. They see the precautions as an expression of paranoia and wasteful spending.”

Kelly and the NYPD, for instance, have come under mounting criticism in the press for pushing the edge of the legal envelope in order to conduct surveillance operations. “We use our investigative capacity to monitor things,” he says, “but always within the law.”

Earlier this week Cathy Lanier, the chief of the Washington D.C. Metropolitan Police, told The Daily Beast she was grateful to Kelly and the NYPD for the extensive intelligence work they conduct, much of which is shared very quickly with other law-enforcement agencies that have fewer resources. “As my grandmother used to say, ‘You are going to be damned for doing and damned for not doing, you better be damned for doing. When the bad thing happens and lives are actually lost, the public won’t forgive you for that.”

In Toulouse, the questions about what was known, and what should have been known, have just begun.