LONDON — They were two terror cells united by, if nothing else, the same target: the London transport system.

By the time the first bombers reached their targets on the morning of July 7, three of them had traveled 200 miles.

Exactly two weeks later, the second group struck just 200 yards from a ground-floor apartment where one suspect lived with his wife and three children.

The two groups of young men were separated by more than miles. Three of the July 7 bombers were British-born sons of a Pakistani immigrant community that has carved out a solid life in the weary industrial neighborhoods of Leeds in Britain's north. One was a schoolteacher; another worked in his father's fish-and-chips shop.

In contrast, at least three of the July 21 suspects came to Britain hardened by youthful odysseys out of the despair of Somalia, Eritrea and Ethiopia. They never got past the margins of British society, sliding into a gray world of unemployment, welfare, public housing and crime.

Yet they came to the same violent conclusion: wreaking havoc in the capital by attacking three Underground trains and a bus with backpack bombs. The first plot killed 52 people, including the bombers. The second could have inflicted the same carnage, but the explosives failed to fully detonate because of a bomb-maker's mistake, police say.

Police were trying to make sense of the differences and similarities alike yesterday as they questioned four captured July 21 suspects and another London man thought to be a possible fifth would-be bomber.

Detectives confront a fundamental mystery. Were the two plots part of a terror campaign by an international network that cobbled together multi-ethnic cells from different cities and disparate backgrounds? Or was the second strike a copycat attack by an unrelated group, as one of the July 21 attackers claimed in a confession to Italian police Friday?

Despite the confession and the contrasts between the Leeds and London cells, most investigators think there has to be a link because of the remarkably similar explosives, targets and methods.

"I think there's plenty of linkage," said a British anti-terror official who asked to remain anonymous. "Two attacks within two weeks with very similar modus operandi. The significance of that alone shouldn't be discounted. What has to be determined is how those linkages work."

British anti-terror agencies are also struggling to understand the threat embodied by two homegrown cells that got past their defenses.

"We are looking at this terrible problem of groups that are not really run from anywhere," said Charles Heyman, a defense expert with Jane's Information Group. "You have local offshoots drawing from al-Qaida doctrine. They are operating almost totally independently, and that makes the intelligence picture very difficult. You've got to have indicators for intelligence, especially electronic indicators. They just aren't there. They are getting under and around the intelligence screen."

Although London's police chief has said the attacks came "out of the blue," at least one bomber in the July 7 attack had surfaced in the investigation of a foiled plot last year involving a network spreading from Pakistan to Britain to North America. He and two others had traveled to Pakistan, where investigators think they were radicalized and possibly trained by al-Qaida operatives.

Meanwhile, the July 21 suspects are products of militant circles centered on the Finsbury Park mosque in north London, a notorious crossroads of extremism. Their alleged leader was a convicted robber and jailhouse convert.

A fugitive would-be bomber captured in Rome on Friday lived only 200 yards from the Stockwell Underground station in the heart of south London's Afro-British community. That station became the launching pad from which three of the bombers embarked on murderous missions: One bomb partially detonated just minutes after a train left the Stockwell platform.

The choice of target seems a defiant statement on home turf, evoking a kind of fierce street-gang mentality as much as Islamic terrorist ideology. They didn't just attack their own country; they attacked their own neighborhood.

In his confession in Rome, the Ethiopian-born suspect insisted that he was part of a self-contained, improvised group with no links to the Leeds crew.

The suspect, 27, was first identified publicly with the alias Hussain Osman, but a senior Italian anti-terror official said his real name is Hamdi Isaac. In his confession as described by the official, the suspect said the four militants had been fired up by the war in Iraq and other jihadi causes. Inspired by the deadly July 7 attacks, they decided to carry out a follow-up strike, prepared their own bombs and followed the script of the previous attacks, according to the confession.

On the other hand, the Rome suspect may not have known of connections to the Leeds cell because that is terrorist tradecraft. The networks are loose and protean, stealthily crafting new networks out of the remnants of old ones. The investigation has to seek a figure capable of orchestrating two cells in isolation from each other.

In Britain, the search for links between London and Leeds could focus on several figures. There is Germaine Lindsay, the fourth dead bomber from July 7. As a Jamaican convert living in Luton, he could have been a nexus between the British-Pakistanis who died along with him and the Afro-British militants now under arrest.

There is also the mysterious Haroon Rachid Aswat, an accused al-Qaida figure with longtime ties to extremist circles in London and the Leeds area. Investigators are trying to determine if he was in telephone contact with some of the Leeds bombers. There are doubts about his role, but his travels and contacts — from Oregon to Africa to Afghanistan — make him intriguing. He has been detained in Zambia.

Another mystery: the bomb-maker. Because the British police retrieved five mostly intact backpack bombs July 21, they have a wealth of forensics evidence. And British security services are still investigating an Egyptian chemist who was arrested in Cairo on July 15 because he had rented the apartment in Leeds used as a bomb factory. Magdy el Nashar, 33, remains in custody and under suspicion, the British anti-terror official said Friday.

"I don't think we should discount him," the British official said. "It is a curious coincidence of location and expertise. There is interest."