It transcends ethnic lines and national causes, blends ideological fervor with common criminality and is rooted to a large extent inside the target country. Shifting assumptions about the nature of the terrorist threat, it also complicates efforts to devise strategies to combat it.

Although some senior intelligence and law enforcement officials said they began to recognize the mutating threat at the time of the train bombings in Madrid in March 2004, the London bombings have reinforced the lesson that, by all accounts, the centrally controlled Al Qaeda of 9/11 is no more.

"We are seeing a terrorist threat that keeps changing," said Pierre de Bousquet, the director of France's domestic intelligence service, known as the D.S.T., in an interview in Paris. "Often the groups are not homogeneous, but a variety of blends."

"Hard-core Islamists are mixing with petty criminals," he added. "People of different backgrounds and nationalities are working together. Some are European-born or have dual nationalities that make it easier for them to travel. The networks are much less structured than we used to believe. Maybe it's the mosque that brings them together, maybe it's prison, maybe it's the neighborhood. And that makes it much more difficult to identify them and uproot them."

In the case of the London attacks of July 7 that left 56 people dead, including the four bombers, three of the attackers were ethnic Pakistanis born in Britain, the fourth a British citizen and convert to Islam born in Jamaica.

The strike that followed two weeks later, in which the four bombs did not explode, was carried out by an intriguing crew that the police say included a British resident born in Somalia, an Ethiopian who apparently posed as a Somali refugee to gain legal residency in Britain and a British citizen born in Eritrea who acquaintances say was radicalized in prison. The nationality and legal status of the fourth would-be bomber has not been disclosed.

The police still say they have not found conclusive evidence linking the two attacks, although the explosives used in both cases, as well as other elements of the episodes, appear to be similar.

None of those identified so far as being involved in the two attacks are believed to have been a battle-hardened veteran of Chechnya or Iraq, and most of them are too young to have been trained in Qaeda camps in Afghanistan, which were destroyed in 2001. They may have learned their bomb-making techniques and terrorist strategies at home, investigators and intelligence officials say, although the officials caution that they do not yet know the extent of the support network behind the attacks or whether either involved a foreign mastermind.

Britain's most senior counterterrorism official himself anticipated what was happening over a year ago. In a little-noticed speech to a conference in Florence in June 2004, Peter Clarke, the counterterrorism chief of Britain's police force, pointed out "the complete change, the recalibration" that Britain was making in investigating the new threat.

The shifting nature of the threat was made apparent early last year with Operation Crevice, one of Britain's largest counterterrorism operations ever, Mr. Clarke said. Seven hundred officers thwarted what they believed was a plot to construct a large bomb intended for a site somewhere in London. In more than two dozen police raids, more than half a ton of ammonium nitrate fertilizer, which can be used in making bombs, was seized and eight ethnic Pakistani British citizens were arrested.

"Before this there was the perception that the international terrorist threat was something that came from abroad," Mr. Clarke said in the speech. "It came from the Maghreb. It came from the Middle East. It came from Chechnya. It came from Afghanistan. These individuals, however, were all British citizens."

"The parameters," he said, "have changed completely."

"If we take one or two leaders away," he added, "very quickly they are replaced and the network is reformed."

He called the homegrown trend "deeply worrying." Equally worrying, he added, was that the "key conspirator" in the plot revealed by Operation Crevice was only 22 years old, and that others were 18 and 19.

A confidential British government assessment of the emerging threat from young British Muslim radicals, prepared last year for Prime Minister Tony Blair, concludes that poverty is not an indication of radicalism, that students and young professionals from working- and middle-class backgrounds "have also become involved in extremist politics and even terrorism." Those recruits, the report warns, "may have the capability for wider and more complex proselytizing."

Extremist organizations have set up outlets on university campuses and, if banned, simply open up again under different names, said the document, whose contents were first disclosed in The Sunday Times. The document divides young extremists into two broad categories. The first category is "well-educated undergraduates" and those "with degrees and technical professional qualifications in engineering" or information technology. The second is "underachievers with few or no qualifications, and often a criminal background."

In particular, the report said, "Muslims are more likely than other faith groups to have no qualifications (over two-fifths have none) and to be unemployed and economically inactive, and are over-represented in deprived areas."

The idea that the terrorist threat is increasingly homegrown and transcends both ethnicity and direct links to a global Qaeda conspiracy is welcomed by Pakistan, which has been accused of not doing enough to root out the remnants of Al Qaeda. Three of the four bombers in the first London attack were of Pakistani descent and at least two had spent time in Pakistan.

"When the first bombing happened and everyone focused on Pakistan, we said, 'You may be making a mistake if you have a unifocal view,' " said Maleeha Lodhi, Pakistan's ambassador to Britain, in an interview. "It's much more mixed up than people think. What you're seeing is something very lethal and it has nothing to do with ethnicity."

"We are seeing a lot of local groups that seem to have a random pattern, no operational linkage or even inspirational linkage," she said. "Some may claim to be Al Qaeda, some not, and that is foxing everybody."

Earlier attacks reflected some of the same elements found in the London bombings. First came Casablanca, then Madrid.

In May 2003, a dozen young, poor, undereducated men, all born and reared in the same slum in Casablanca, Morocco, attacked five sites there, four apparently chosen for their Jewish connections. Forty-two people died, including the attackers.

"It was local guys thinking global," said Olivier Roy, author of the book "Globalized Islam."

"They didn't target a symbol of the Moroccan government," he added. "They inscribed their actions in a global perspective. I'm not sure the ethnic Pakistanis involved in the first London attacks have anything to do with Pakistan."

The train attacks in Madrid in March last year represented more of a blend. While most of those involved were Moroccan, some were from other countries. Some of the attackers were radicalized Muslims, others common criminals.

The most senior member of the team, and the suspected local leader of the cell, was a Tunisian who aspired to be a fashion model but became a successful real estate agent before turning radical.

The Madrid plotters included native Spaniards, who had no connection to global jihad, including a former miner who was arrested on charges that he stole and handled the explosives used in the operation and a 16-year-old nicknamed "The Gypsy" who was given a six-year youth detention sentence last November after pleading guilty to transporting explosives. In searching for the mastermind of the Madrid attacks, the Spanish authorities have focused on a number of foreign-based suspects, including an Egyptian and a Syrian.

In London, investigators are trying to determine whether the cells involved in the attacks were homegrown or had any operational link to a wider network.

Investigators say that while they see the terrorism threat in Europe as more homegrown, the inspiration is increasingly Iraq. In the past several months, a number of European countries have uncovered cells of native-born men poised to travel to Iraq to fight alongside the insurgency.

In an interview published in Le Parisien on Friday, Interior Minister Nicolas Sarkozy of France said at least seven Frenchmen had been killed while fighting with the insurgency in Iraq.

The ever-shifting nature of the threat has made it increasingly challenging, in Britain and elsewhere, to come up with a strategy to combat it. Police and intelligence officials acknowledge that they are still too focused on threats linked to clearly identifiable ethnic radical groups, both domestic and international, and not enough on homegrown blends.

In a cover letter to the 2004 British report on counter-terrorism, Sir Andrew Turnbull, the cabinet secretary and one of Mr. Blair's closest aides, said the goal of Britain's strategy was "to prevent terrorism by tackling its underlying causes, to work together to resolve regional conflicts to support moderate Islam and reform and to diminish support for terrorists by influencing relevant social and economic issues."

But, he added, "without being clear about the nature of the problem, one can only tentatively identify possible responses in general terms."

Hélène Fouquet contributed reporting for this article.