LONDON — British and Saudi investigators are examining a series of phone calls, text messages and e-mails between leaders of the al-Qaida 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 calls, said the official, who spoke on condition of anonymity, and they are now part of the ongoing investigation. British officials declined to comment on the disclosure.

The possible Saudi connection is one of several lines of inquiry investigators are following in their continuing hunt for those responsible for two sets of recent attacks in London — the July 7 bombings of three subways 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 refused to be identified, 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 yesterday in connection with the attacks, while the authorities said they would formally apply today 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 scene. 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 sweeping anti-terrorism laws, they can be held for up to 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 whitewater-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, is believed to have left behind on an East London bus. Two of the July 7 bombers from Leeds had participated in rafting at 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-based bombers, Mohammed Sidique Khan, 30, also frequented the mosque before it was seized by the authorities and its radical imam, Abu Hamza Masri, charged with encouraging his followers to kill non-Muslims. But no one has yet connected Khan to the other two men.

In statements to interrogators in Rome, Hamdi has claimed 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 after the earlier attacks.

Police want to determine if Khan and one of his fellow alleged bombers, Shehzad Tanweer, 22, received instructions to carry out their attacks on July 7 — which marked the opening of the Group of Eight meeting of world leaders in Scotland — as well as training in bomb making from al-Qaida 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 cellphones of Abdul Karim Majati, a Moroccan believed to be the head of the al-Qaida 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 cellphones tied to Younis Mohammed Ibrahim Hayari, another Moroccan al-Qaida leader, and his associates. Hayari, who headed 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-Qaida 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 newspaper reported Hamdi made a phone call to Saudi Arabia shortly before he was arrested Friday. The London Sunday Times reported Ibrahim, the July 21 group's reputed ringleader, visited Saudi Arabia in 2003, telling friends he went there for 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.