The bombings in London highlight a post-Sept. 11 trend in which extremists have been going after easy-to-hit "soft targets" with the aim of inflicting mass casualties, undermining confidence in governments, and drumming up new recruits and support.
Some U.S. officials and independent experts attribute the trend in large measure to changes that extremists have made in their tactics since President Bush launched the U.S.-led global campaign against terrorism.
The campaign led to stiffened security precautions and new laws worldwide and better international counterterrorism cooperation and intelligence sharing. These measures left many senior al-Qaeda operatives and thousands of adherents dead or in custody, and they hampered groups' fund-raising and communications.
Deprived of his sanctuary in Afghanistan, Osama bin Laden remains on the run, and his followers and other extremist groups are the targets of massive manhunts.
As a result, it's become far harder for them to pull off complex operations involving large numbers of participants and sums of money moving from continent to continent, U.S. officials and experts say.
In response, they said, al-Qaeda has metastasized into a worldwide movement of small cells loosely linked to, or inspired by, bin Laden.
Instead of Sept. 11-style operations that take years to plan, these groups have been mounting strikes requiring much less time, much less money, and far fewer people against public places and gatherings and other unguarded or lightly protected targets.
"Part of the reasons that today's al-Qaeda is different from the old al-Qaeda is that Osama bin Laden no longer has freedom of movement in a liberated zone," said John Pike of www.globalsecurity.org, an Internet clearinghouse for information on defense and intelligence issues.
Joe Morton, the acting head of the State Department bureau that oversees security for U.S. embassies, said at a conference in May: "We've witnessed... a shift in the type of targets that terrorists have been choosing.
"Though al-Qaeda has been weakened operationally, it has adapted by spreading its ideology to local groups throughout the world. These extremist regional groups are conducting attacks that are more local and less sophisticated but still very lethal."
Al-Qaeda is the leading suspect in the bombings that killed at least 50 people in the London Underground and on a bus Thursday.
Attacks on easy-to-hit targets also have become a daily feature of the war in Iraq, where bombers, many dispatched by al-Qaeda's Iraqi affiliate led by Abu Musab al-Zarqawi, have been hitting hotels, restaurants, and other public buildings.
In Afghanistan, a resurgent Taliban militia and al-Qaeda followers may now be pursuing a similar strategy against the U.S.-backed government there. Attacks on soft targets have been on the rise. Recent targets include an Internet cafe and a funeral.
Strikes on soft targets have also been confounding governments from Indonesia, where 202 people died in October 2002 bombings in the tourist resort of Bali, to Spain, where commuter-train bombings in March 2004 killed 191 people, prompting the government's electoral defeat and a Spanish troop pullout from Iraq.