British and Saudi investigators are examining a series of phone calls, text messages and e-mails between leaders of the al Qaeda network in Saudi Arabia and unknown people in Britain from February to May for possible links to the recent bomb attacks in London or a still unidentified group of extremists operating in Britain, according to a Saudi official.

After the July 7 bombings of London's transit system that claimed 56 lives, the British requested further information about the communications, said the official, who spoke on condition of anonymity, and they are now part of the investigation. British officials declined to comment on the disclosure.

The possible Saudi connection is one of several lines of inquiry investigators are following as they seek to make progress in their hunt for those responsible for two sets of recent attacks in London -- the July 7 bombings of three subway trains and a double-decker bus, and an abortive attack two weeks later in which assailants failed to detonate explosives on an identical combination of three subway trains and a bus.

Despite their success last week in rounding up all of the suspects in the failed July 21 attacks, investigators concede they have not answered several key questions: Were the two sets of attacks linked? How were they planned and financed? Was there a larger network of extremists, domestic or foreign, behind the bombings? And, most crucially, are there more attacks in the pipeline?

"We're very pleased with what we managed to achieve last week," said a British official who spoke on condition of anonymity, in keeping with government custom. "But there's so much more we need to find out."

Police in the seaside city of Brighton seized six men and a woman on Sunday in connection with the attacks, while authorities said they would formally apply on Monday for the extradition from Italy of Isaac Hamdi, also known as Osman Hussain, one of the suspects in the July 21 attacks.

The four suspects in the July 7 attacks all died in the bombings, while those allegedly responsible for the botched July 21 attacks fled the scenes. After receiving tips from the public, police seized one of the suspects Wednesday and swooped down on two more Friday, while authorities in Rome arrested Hamdi, who had sought refuge there. Another man was arrested in London in connection with a fifth bomb that was abandoned unexploded in a west London park.

All of the men are being interrogated at a high-security police station. Under Britain's anti-terrorism laws, they can be held for as long as 14 days without charge.

Both sets of attackers were young Muslims with a growing sense of rage over Britain's participation in U.S.-led military operations in Afghanistan and Iraq, officials say. Three of the four men behind the July 7 attacks were British nationals of Pakistani origin who were born and raised in the northern city of Leeds; the fourth was a Jamaican-born convert to Islam who also lived near Leeds for a time. The July 21 suspects were all London-based men of East African origin.

The only tangible link between the two sets of bombers, according to officials, is a brochure for a white-water rafting center in northern Wales. The brochure was discovered in a backpack containing undetonated explosives that one of the alleged July 21 attackers, Muktar Said Ibrahim, 27, left behind on an east London bus. Two of the July 7 bombers from Leeds had participated in a rafting trip with the center in early June.

It has also been reported that Ibrahim and another suspect, Yasin Hassan Omar, 24, were devotees of the Finsbury Park Mosque in north London, once a hotbed of Islamic extremism. One of the Leeds bombers, Mohammed Sidique Khan, 30, also frequented the mosque before it was seized by the authorities and its radical imam, Abu Hamza Masri, was charged with encouraging his followers to kill non-Muslims. But no one has yet connected Khan to the two other men.

Investigators believe that both sets of bombers used a homemade concoction of explosives made from triacetone triperoxide, a volatile household material. But the Leeds bombers made their bombs in the bathtub of a flat in the city, while the July 21 bombers are believed to have made theirs in an apartment in north London where Ibrahim and Omar lived.

In his statements to interrogators in Rome, Hamdi has asserted that there was no connection between the two sets of attackers, the Corriere della Sera newspaper reported, but that the July 21 would-be bombers decided to "take revenge on the English" for the anti-Muslim atmosphere following the earlier attacks.

"People gave us bad looks and made fun of us in the street, even women were mocked," said Hamdi, according to the newspaper. "We decided to react."

"We didn't have plans for afterwards," added Hamdi, who grew up in Rome and speaks fluent Italian. "Then I thought about going to Italy to my brother's place, but without explaining anything to him. It was only when we saw each other that I confessed to him that I was one of the people they were looking for in connection with the July 21 bombings."

Police are seeking to determine whether Khan and one of his fellow alleged bombers, Shehzad Tanweer, 22, received instructions to carry out the July 7 attacks, as well as training in bomb-making, from al Qaeda operatives in Pakistan. Both men visited that country late last year. So far, officials said, they have found no clear evidence to support that theory.

Still, they believe a foreign link exists. "We're looking for a mastermind because there's always a mastermind in these attacks, and we think he probably came from somewhere else," said a senior British official.

The Saudi connection is one possibility investigators are exploring. Saudi investigators traced the suspect communications and then informed their British counterparts, but did not hear back about what action, if any, the British took. "We said check out these numbers," the Saudi official said.

He said the calls to and from Saudi Arabia were linked to prepaid cell phones of Abdul Karim Majati, a Moroccan believed to be the head of the al Qaeda network in the Persian Gulf area, and to his associates. Majati was killed in April by security forces.

The calls between London and Saudi Arabia also involved cell phones tied to Younis Mohammed Ibrahim Hayari, another Moroccan al Qaeda leader, and his associates. Hayari, who topped Saudi Arabia's most wanted list, was killed July 3 in a shootout with Saudi security forces.

In some cases, the text messages used aliases to transfer money through a series of personal transactions, the official said. Al Qaeda has long used that system of moving money, known as hawala.

The calls dropped off in May, the official said, but are the center of renewed attention because of the July attacks here.

Tentative links between the London bombers and Saudi Arabia have begun to emerge. The Sunday Telegraph reported that Hamdi made a phone call to Saudi Arabia shortly before he was arrested Friday. The London Sunday Times reported that Ibrahim, the reputed ringleader of the July 21 group, visited Saudi Arabia in 2003, telling friends he went there to receive training.

The Saudi official confirmed both the phone call and the visit. He also said Saudi investigators were examining the travel of one of the July 7 bombers, Hasib Hussain, who transited through Riyadh, the Saudi capital, in 2004 on his way to Karachi, Pakistan.

In the days immediately after the July 7 attacks, British investigators searched for a man they believed had entered the country two weeks before the bombings, contacted Khan by phone, then left the country hours before the attacks. At one point, unnamed American counterterrorism officials identified Haroon Rashid Aswat, 30, a British national who is wanted in the United States for allegedly seeking to establish a terrorist training camp in Oregon in 1999, as the suspect and possible organizer. But British officials have since insisted they have found no evidence that Aswat, who grew up in the Leeds area and is reportedly a resident of South Africa, was in Britain before the attacks took place.

Zambian police arrested Aswat 10 days ago after he entered the country and are holding him pending an extradition request from the United States. British officials have said they would like to speak to Aswat about his reputed al Qaeda connections.

Officials have cautioned the British public to remain on alert for further attacks. Press reports said officials were planning to place teams of officers at every subway, rail and bus station on Thursday, two weeks after the July 21 botched attacks. Each of the two previous attacks, spaced two weeks apart, also occurred on a Thursday.

Special correspondent William Magnuson in Rome contributed to this report.

A tarpaulin covers a London apartment following arrests there in connection with the investigation into attacks in the city. Police warned the public to remain alert.

Police officers in London consult outside residence where the arrests were made.