WASHINGTON, July 12 - A suicide bombing may be the ultimate act of devotion, and so it also represents the ultimate security nightmare: an attack in which the one essential ingredient is not training or technology but commitment.
It is a tactic that has proved so effective in cities like Beirut, Jerusalem and Baghdad that it would only be a matter of time, security experts have warned, before it would migrate to other parts of the world.
In London, that moment may have arrived with the police announcement that one bomber had died in the wreckage and property belonging to three others had been found at the locations of the other blasts.
If the attacks in London do prove to have been suicide attacks, the outlines of life could change, with new fears of copycat attacks and a new awareness of the impotence of measures like immigration checks and public address announcements urging a lookout for suspicious parcels.
"A suicide attacker could be anyone," said Daniel Benjamin, a former Clinton administration official and terrorism expert who is the author of "The Next Attack," due to be published this fall. "He doesn't have to be trained, just indoctrinated. There's no profile; that's what makes it so hard to defend against."
Of course, Americans have already been confronted - on Sept. 11, 2001 - with attackers willing to die for their cause. But that plot required hijackers, jetliners and a choreography far more complex than the chillingly simple steps involved in boarding a subway train or a bus and detonating an explosive, as at least one of the attackers from Leeds appears to have done in London last week.
Why suicide attacks have not previously emerged in the West is a mystery. In the Middle East and in Asia, the tactic has spread in recent years far beyond its origins in Lebanon in 1982, where it was pioneered by the Shiite Muslim group Hezbollah.
In Israel and its occupied territories, Palestinians have carried out more than 200 suicide attacks in the last decade, killing hundreds of Israeli soldiers and civilians. In Iraq, since the American invasion of 2003, more than 500 suicide bombings have been carried out by Iraqi insurgents and foreign fighters aligned with them, at a toll of hundreds and hundreds of Iraqi and American lives.
But with the notable exception of Sept. 11, the same pattern has not emerged in the West, the nominal target of most Islamic extremists. In March 2004, the terrorists who killed 191 people in Madrid detonated their explosives remotely, and only killed themselves when the authorities had surrounded their hide-out.
Even last week, after the London attacks, a senior American intelligence official cast doubt on the idea that suicide bombers could have been responsible, saying that initial reports to that effect "have not been confirmed at all."
In offering explanations for why Western shopping malls and nightclubs have not yet been touched by suicide attacks, American officials have suggested that this may represent a success for immigration controls and law-enforcement measures that have put a premium, particularly since Sept. 11, at screening out Islamic extremists.
Michael Chertoff, the new homeland security chief, said in an interview on ABC News over the weekend that while some suspects had been charged only with minor crimes, their arrests might have pre-empted larger operations.
American officials have also suggested that whatever remains of the core of Qaeda, headed by Osama bin Laden and his lieutenants, probably has little interest in squandering its resources on a small-scale attack. The bombings in London appear to have claimed fewer than 100 lives, compared with the nearly 3,000 killed in the Sept. 11 attacks, and most American intelligence officials have long said they believed that Al Qaeda remained determined to surpass Sept. 11 in scope.
In public testimony earlier this year, Porter J. Goss, the director of central intelligence, said that it "may only be a matter of time before Al Qaeda or another group attempts to use chemical, biological, radiological and nuclear weapons."
Still, Mr. Goss also noted that Al Qaeda "is only one facet of the threat from a broader Sunni jihadist movement."
For smaller homegrown groups, like the one that acted in Madrid and whatever operation may have been behind the London attacks, the daily example of the havoc that can be created by suicide attacks in places like Iraq and Israel may have proved impossible to ignore.