The British-born man in his 30s, of Pakistani origin, arrived at a British port last month and left the country again the day before Thursday's attacks, The Times newspaper reported.

The four suspected suicide bombers, three of whom have been identified by newspapers, were also Britons of Pakistani origin. UK security sources believe the mastermind was involved in previous terror operations and has links with followers of Osama bin Laden's al Qaeda extremist group in the United States.

He is thought to have visited the bombers in their home city of Leeds, northern England, and identified targets on the London Underground rail subway system where three of the four bombs exploded, the paper said.

It is thought that he was likely to have trained the recruits in how to trigger their rucksack-carried bombs, three of which went off almost simultaneously at around 8:50 a.m. (0750 GMT), at the peak of last Thursday's morning rush hour killing at least 52 people.

Police have reportedly switched their investigation towards
seeking the mastermind of the bombings since learning that the four bombers killed themselves in the attacks.

Home Secretary Charles Clarke said earlier Wednesday that the aim was now "to attack the people who are driving, organizing and manipulating" bombers.

Police are also seeking a possible fifth member of the bombers' terror cell, who was also seen at Luton station, north of London, from where the attackers traveled into the capital. The man, also believed to be of Pakistani origin, could still be at large in London, The Times said.

Egyptian sought

Police additionally want to interview an Egyptian-born university lecturer who was teaching in Leeds until a few weeks ago. According to The Sun newspaper, the man, who it named as 33-year-old Magdi El-Nashar, was studying for a biochemistry doctorate at Leeds University and disappeared just before the attacks, it is thought to Egypt.

At least two of the bombers had links to his rented flat, which is one of six addresses in and around Leeds raided by police on Tuesday morning.

"A detailed forensic examination is expected to be carried out and may take some time to complete," a statement from London's Metropolitan Police said, adding that there had been no arrests from the raids.

The suspects were named in newspaper accounts as Shehzad Tanweer, 22, Hasib Mir Hussain, 19, Mohammed Sadique Khan, 30, and Eliaz Fiaz, 30.

Britain was set to fall silent for two minutes on Thursday to remember the victims of the London suicide bombings. People have been asked to stop what they're doing at midday (1100 GMT), and pay silent respect to the people who died last Thursday. There will be similar commemorations across Europe, including in Germany.