WASHINGTON — Symbolic, strategic, catastrophic.

The London attacks bear the hallmarks of a sophisticated terror group: precise planning, bombs set to explode in succession at a time and place — rush hour in a mass-transit system — and calculated to cause mass casualties and panic.

Though it is too early to pinpoint the true perpetrators — despite an early claim of responsibility — experts believe that a group in the wider "nebula" of the al-Qaida organization may well be responsible. Moreover, London, they said, has increasingly become a center for indoctrination and recruitment of Islamic extremists and, inevitably, a target itself.

"The attack is symbolic because of the G-8 meetings, strategic because it targeted mass transportation and catastrophic in the number of casualties — a methodology consistent with attacks by al-Qaida," said Gerry Leone, a former Massachusetts federal prosecutor who helped convict al-Qaida "shoe bomber" Richard Reid.

"It has the hallmarks of al-Qaida, and we know the al-Qaida movement is increasing in Europe and the Gulf," said terrorism expert Jessica Stern, a Harvard University lecturer and author of "Terror in the Name of God."

"They are targeting Europe as a source of recruits among a growing number of Islamists and as a potential place for attacks."

The bombings of three subway trains and a double-decker bus at rush hour also illustrate one of the great frustrations of the war on terror: It's nearly impossible to defend against assailants who use relatively simple technology to shock and disrupt civilian life, U.S. officials and counterterrorism experts say.

"These attacks are a grim warning that ... even the most experienced anti-terrorist force cannot provide security in open democratic societies," said security expert Anthony Cordesman of the Washington-based Center for Strategic and International Studies.

While al-Qaida's core organization — sometimes known as "al-Qaida central" — has been on the run since the Sept. 11, 2001, attacks and stands denuded of much of its leadership, local groups inspired by it have popped up across the Middle East, North Africa and Europe.

Britain has long experience with terrorism from its three-decade battle with the Irish Republican Army, and its security services have disrupted several alleged plots by Muslim radicals over the last year and a half.

There were no definitive answers yesterday as to who was behind the London bombings.

A previously unknown group, calling itself the "Secret Organization of al-Qaida in Europe," claimed responsibility in an Internet posting and warned Denmark and Italy to withdraw their troops from Iraq. The claim couldn't be immediately verified.

A similarly named group, al-Qaida in Europe, claimed responsibility after the March 11, 2004, Madrid commuter train bombings, which killed 191 people.

Stephen Ulph, an Islamic affairs analyst for British-based Jane's Information Group, said the style and language of the letter on the Internet were similar to that of previous al-Qaida announcements.

This was "probably a propaganda attack. ... To say 'We're still here,' " Ulph said But Ulph, who's affiliated with the Washington-based Jamestown Foundation, said the attacks were a failure strategically. "No great panic ... was caused by this," he said.

Beginning in 2003 and intensifying after the attacks on the Madrid commuter rail system, Londoners have been told repeatedly to prepare for similar attacks in their hometown. Again and again, the London Underground train system was described as "vulnerable."

London emergency workers held practice runs on how to respond to a terrorist attack on the subway. During the exercises, stretches of the Underground would be shut down, as police cadets pretended to be victims of an attack.

Police have broken up several plots. Last April, British security forces derailed a plot to flood sections of the subway with poison gas, or a so-called radioactive "dirty bomb." Police discovered a plan to release osmium tetroxide within the confines of the subway system, which officials said could have resulted in many deaths. Information on the alleged planned attack seemed to indicate a possible al-Qaida link.

Last August, British authorities also uncovered an alleged plot to set off a radioactive dirty bomb in London; they charged eight men with terrorism-related crimes. One of them was Dhiren Barot, al-Qaida's reputed leader in Britain, who was alleged to have surveillance plans of buildings in New York, New Jersey and Washington that were the subject of a U.S. terror alert nearly a year ago.

London is also home to the Finsbury Park Mosque, where alleged Sept. 11 plotter Zacarias Moussaoui and "shoe bomber" Richard Reid worshipped. A radical cleric who used to preach there, Abu Hamza al-Masri, is on trial on 15 terrorism charges.

Stephen Gale, a counterterrorism expert at the Philadelphia-based Foreign Policy Research Institute, said London has "probably the largest concentration of al-Qaida sympathizers and cells in Western Europe."

But as to the attackers, Gale said, "Right now, we have no idea. And that's part of the problem. It's all ambiguous."