British Intensify Terror Probe

Britons marked the first week since the London bombings with a

The one week mark arrived as a person injured in the blast on the No. 30 bus in Tavistock Square died in a hospital, bringing the death toll to 54.

The man was not identified, and no other details were immediately released.

Metropolitan Police Commissioner Ian Blair told the Foreign Press Association that police believe "that we know who the four people carrying the bombs were ... and we believe they are all dead."

"We are as certain as we can be that four people were killed and they were the four people carrying bombs," Blair said.

His comments were the first public confirmation from police that the July 7 attackers were suicide bombers. Bombs exploded on three subway trains and a double-decker bus, killing at least 53 people, including the attackers.

Peter Clarke, head of the Metropolitan Police anti-terrorist branch, on Thursday identified the suspected suicide bomber who blew up the double-decker bus, killing 13 people, as Hasib Hussain, 18. Clarke also said Shahzad Tanweer, 22, was responsible for attacking a subway train between the Liverpool Street and Aldgate stations. Both are Britons of Pakistani descent.

News reports have identified the other two as Mohammed Sidique Khan, 30, another Briton of Pakistani descent, and Lindsey Germaine, a Jamaican-born Briton.

Blair declined to comment on those reports, and he would not say how many suspects are being sought.

"We don't know if there is a fifth man, or a sixth man, a seventh man," he said, but added that police were trying to determine who organized the attack.

It is becoming more clear that Siddique Khan was central to the conspiracy, CBS News Correspondent Sheila MacVicar reports. At 30, he was the oldest of the bombers, and used his skills with young people to recruit Hussein and Shahzad Tanweer.

He was a respected teacher who lived a double life as a militant.

In an interview with the BBC, a family friend whose identity was protected for his own safety talked of Khan's frequent trips to extremist training camps in Pakistan.

Two claims of responsibility for the bombings, purportedly from militant Islamic groups, have surfaced.

Commenting on the possible role of al Qaeda, Blair said, "al Qaeda is not an organization. Al Qaeda is a way of working ... but this has the hallmark of that approach."

MacVicar reports British investigators are trying to track the movements of an Egyptian-born chemistry student who had lived in Leeds for five years and who is either the bomb-maker or his assistant.

The student completed a Ph.D. at the University of Leeds earlier this year. Investigative sources say that he asked to use an apartment being looked after by an acquaintance, telling him it was "for a friend from London." The Egyptian asked that the apartment be emptied of furniture. The apartment manager never met the mysterious "friend from London," but did speak to someone with a British accent on the telephone.

Police raided the apartment on Alexandra Grove two days ago and, it is reported, discovered bomb-making material and perhaps, explosives, there. The Egyptian student disappeared from Leeds one week before the bombings, and there is a suggestion that the "friend from London" may be a senior al Qaeda operative, a British citizen of Pakistani descent, who investigators suspect arrived in the U.K. shortly before the bombings to bring the plot to fruition.

British Intelligence Services have asked their Egyptian counterparts to find the student. He told people in Leeds he was going to Cairo.

The Daily Telegraph said police were trying to identify a man seen standing near the four suspects on the Luton railway station platform, where they apparently boarded a train for London on July 7.

The Evening Standard reported that police spotted a fifth man on closed-circuit TV of the group at London's King's Cross station about 20 minutes before the explosions.

Meanwhile, Britons and people around the world marked the first week since the attacks with a silent tribute to the victims.

Queen Elizabeth II stood motionless outside Buckingham Palace, and a crowd of thousands filled Trafalgar Square, where many could be seen wiping away tears and hanging their heads in prayer during the two-minute tribute that began at noon.

The silence was broken only by the tolling of Big Ben.

Outside King's Cross station, a focus for the city's grief in the week following the bombings, trucks, cars and mounted police officers paused on busy Euston Road to join the tribute.

In the evening, an estimated 15,000 people returned to Trafalgar Square for a vigil, waving British flags and banners denouncing terrorism. "London has survived bombings, burnings, wars and just about everything history can throw at it," Nigerian-born poet Ben Okri said as the crowd cheered.

In the northern city of Leeds, the home of at least two of the bombers, young Muslims paused in silence before speeches by local imam, a minister and community leaders.

"We condemn these terrorists and what they have done," said Munir Shah, imam of the Stratford Street mosque. "We refuse to call them Muslims. They are not. Islam does not agree or teach about the killing of innocent people."

Earlier, police searched yet another Leeds address in their hunt for anyone who aided the bombers. Authorities suspect the four didn't work alone and that their collaborators or leader are likely still at large.

Prime Minister Tony Blair marked the silence in the garden of his official resident at 10 Downing St., where he hosted a reception for police officers receiving bravery awards not related to the bombings. In the House of Commons and the House of Lords, lawmakers broke off debates to join in the silence.

During the tribute, the usually thronging hordes of tourists outside Westminster Abbey also fell silent. Vehicles halted in the usually busy Parliament Square, as taxi drivers and other motorists ignored the green signals on traffic lights.

"As we stand together in silence, let us send a message to the terrorists you will not defeat us and you will not break us," said George Psaradakis, who was driving the double-decker bus that was bombed, killing 13 passengers.

Across Europe, people also paused to honor the dead.

In Madrid, Spain, which was hit by al Qaeda-linked train bombings that killed 191 people last year, Mayor Alberto Ruiz Gallardon and other officials observed the silence in a plaza outside town hall.

Sirens wailed across Paris, with French President Jacques Chirac and visiting Brazilian President Luis Inacio Lula de Silva standing at attention outside the Elysee Palace.

Pope Benedict XVI prayed for the victims at his holiday retreat in the Italian Alps, the Vatican said.