The bombings that rocked the British capital Thursday closely follow a pattern embraced by Islamic radicals in the past three years in Madrid, Casablanca, Bali and Istanbul: low-tech, but coordinated explosions in crowded public places, designed to inflict as many civilian casualties as possible.

Although investigators are a long way from concluding who was behind the London attacks, they bore an especially strong resemblance to the last major terrorist strike in Europe, the March 11, 2004, train bombings in the Spanish capital of Madrid. In both cases, the bombers targeted largely unprotected public transport systems during the height of the morning rush hour with multiple bombs that blew up within minutes of each other.

"Many things remind me of March 11, unfortunately," said Rafael L. Bardaji, a security adviser to former Spanish prime minister Jose Maria Aznar, who lost his re-election bid in a highly charged vote three days after the bombings. "The modus operandi is basically the same. On the operational side, the similarities are striking."

In Madrid, assailants left 13 backpacks and packages filled with explosives on commuter trains and detonated 10 of them from afar with cellular phones, killing 191 people and wounding more than 1,800. In London, witnesses and investigators said there was no sign of suicide bombers being involved in the three subway explosions, which they said may have been caused by devices that were hidden in bags and left on the trains.

British security officials were investigating the possibility that the perpetrator of the fourth bombing, on a bus near Russell Square, may have been killed in the explosion, perhaps accidentally, according to two sources who were briefed by the officials. All told, at least 37 were reported dead and more than 700 wounded in the London attacks.

Unlike the Sept. 11, 2001, hijackings in the United States, which had been preceded by intelligence reports during the summer that Osama bin Laden and al Qaeda were plotting a major attack in the U.S., there was no immediate sign that officials in Madrid or London had any specific warning or indication that a terrorist strike was imminent, much less advance information on who was behind it.

"Both countries failed miserably in preventing this from happening," Bardaji said. "Such an act requires quite a lot of people. You need many people to prepare the explosives. There are many people in communication with each other. Despite all the efforts by the police and the intelligence services in London, it is a major failure."

Unlike Spanish officials, who had not considered themselves a target for Islamic radicals, leaders in the United Kingdom have known for years that the country faced a serious and deep-seated threat from extremists.

"Britain was always considered a juicy target, given that it has such a close relationship with the United States, both in the war on terror and in Iraq," said Jonathan Stevenson, senior fellow for counterterrorism research at the International Institute for Strategic Studies in London.

For years, he noted, top British officials have warned the public that London in particular was in the crosshairs of radical Islamic groups. Security officials have long seen the London Underground as a likely target as well.

"They've said an attack is inevitable -- they've always used the word 'inevitable' -- and unfortunately they've been proven right," Stevenson said.

On Thursday, hours after the killings in London, a group calling itself the Secret Organization of al Qaeda in Europe posted a statement on the Internet asserting that the bombings were "in retaliation for the massacres Britain is committing in Iraq and Afghanistan."

The statement also predicted more attacks in Europe, singling out Denmark and Italy, both of which have sent troops to Iraq.

Two days after the bombings in Spain, a previously unknown group calling itself "al Qaeda in Europe" asserted responsibility for the attacks in a videotape that was found by investigators. In the videotape, a man who identified himself as the network's military spokesman said the bombings were "a response to the crimes that you have caused in the world, and specifically in Iraq and Afghanistan, and there will be more, if God wills it."

It is unclear if the two groups are related, or even if they played any role in either attack. Spanish investigators arrested dozens of suspects in the Madrid case, including several members of the Moroccan Islamic Combatant Group, a network of radicals affiliated with al Qaeda. But European counterterrorism officials have said they still don't know who gave the order for the Madrid attacks. Nor are they sure if they were conceived locally or directed from elsewhere.

Similarly, Moroccan officials remain unsure about who may have inspired the suicide attacks on Jewish and Western targets in Casablanca in May 2003 that killed 45 people, and Egyptian authorities have not concluded their investigation into bombings at several Red Sea resorts last October that killed 34 people.

Counterterrorism officials and analysts in Europe, North Africa and the United States have said that since the Sept. 11 hijackings, an increasing number of terrorist attacks have been planned and carried out by Islamic radicals who have fleeting, if any, ties with the central al Qaeda organization led by Osama bin Laden.

But according to the officials and analysts, the local groups may be sharing information or receiving direction from organizers with experience in arranging such plots.

"What we're seeing are far more autonomous groups that are inspired by al Qaeda's message of hatred of the West," said Alex Standish, editor of Jane's Intelligence Review and senior research fellow at Durham University in England.

"In the last few years, these groups seem to have adopted a very cellular structure. Most of them have very tightknit cells, with eight to 10 individuals or so, who have personal links that go back for years."

U.S. intelligence agencies began sifting through communications intercepts from the time of the blasts and were re-examining data collected in the past few weeks for clues to Thursday's attacks. Current and former intelligence sources said it was too early to make a definitive judgment but suggested there were "no indications that bin Laden's al Qaeda was behind this," according to one source.

"The good news is that it is difficult for bin Laden to issue orders and direction to people now," another intelligence official said on condition of anonymity. "But the bad news is that it is harder to track and stop inspired groups who leave fewer footprints, communicate less and don't need to travel as far."

U.S. intelligence officials did not dismiss the possibility that Abu Musab Zarqawi, a longtime Islamic radical who has helped direct the insurgency against U.S. forces in Iraq, could have inspired or directed the explosions in London.

Zarqawi's network has recruited fighters from across Europe and has been blamed for planning attacks in Germany and Britain.

CIA Director Porter Goss said in Senate testimony in February that Zarqawi's aim is to use Iraq as a base, as al-Qaeda had done in Afghanistan, and carry out attacks against Western targets outside the country.

Staff writers Dan Eggen and Susan B. Glasser in Washington, staff writer Dafna Linzer in New York and researcher Julie Tate in Washington contributed to this report.

Metropolitan Police officers search a trash can outside Aldgate East station in London after four near- simultaneous explosions rocked the area. Investigators said the style of the attacks recalled tactics used by Islamic radicals. Shocked commuters walk away from Edgware Road underground station where a blast killed seven people. The explosion caused a train to collide into two others.