Mohammed Sidique Khan, 30, had links with an al-Qai'da operative, visited Pakistani religious schools run by terrorist groups and was in Israel in the year that the British Muslims Mohammed Hanif and Omar Khan Sharif arrived to blow up Mike's Place, a jazz bar on the Tel Aviv seafront.
He seems increasingly to be the charismatic "Mr Khan" whose influence the youngest bomber's family voiced fears about in the weeks before the blasts.
Khan was known to Mohammed Junaid Babar, a terrorist in US custody who pleaded guilty last year to providing material support to al-Qa'ida, according to two American intelligence officials. They have said Babar was shown photographs of the four bombers last Thursday and identified Khan as a man he met in Pakistan, from two separate photographs.
The intelligence has not been corroborated by British security services but, if true, would conflict with the initial suggestions that the London bombers were home-grown "clean-skins" completely unknown to security services.
Khan, a one-time primary school teaching assistant from Dewsbury, West Yorkshire, killed six other passengers when he triggered the Edgware Road station bomb. The death toll from the four blasts reached 55 at the weekend.
Khan came to the notice of the police and the security agencies last year over an alleged plot involving a 600lb truck bomb in London but was dismissed as a mere "criminal associate" and no further investigation was done.
It is believed he took part in "low-level" criminality including credit card fraud. Investigators now believe the money raised was used for political and terrorist-related activities, something not foreseen at the time.
Khan's work as a primary school teaching assistant enabled him to join a school delegation to the House of Commons, 12 months ago. But he also went to Pakistan on more than one occasion, visiting madrassas (religious schools) run by Jaish-e-Mohammed, a group that has staged terrorist attacks in Kashmir and has ties to al-Qa'ida.
The group was allegedly banned by Pakistan after pressure from the US but continues to operate under different names. Khan is also said to have attended a madrassa run by the group Jamat-e-Islam, which is legal and has a huge network of ancillary organisations within Pakistan.
Khan's links to the Mike's Place bombing remain uncertain. Israeli security services refused to confirm or deny reports in Ma'ariv, an Israeli mass-circulation daily paper, that he visited Israel in 2003. But informed sources believe he did help plan the bombing. British security sources said they were unaware that Khan had been in Israel at the time.
The Israeli secret service, Mossad, is understood to be sharing information with its British counterparts. According to Israeli media, another British-born terrorist, the "shoe-bomber" Richard Reid, also visited Israel on a reconnaissance mission before he tried to blow up an airliner bound for Miami in December 2001. Bashir Ahmed, Shahzad Tanweer's uncle, said Khan "groomed" his nephew in a gymnasium below the Hardy Street mosque near the family's home in Beeston, Leeds. "It was below the mosque and the only adult inside was Khan. At the time, no one had a problem [with that] because he was a respected teacher," he said.
The gymnasium's equipment was funded through two £2,000 European aid grants awarded by Leeds City Council, though the council said that the local Kashmir Muslims Welfare Association and not Khan had applied for the grant.
Khan's emergence as a central figure behind the bombings is extraordinary, given the six months he once spent advising youngsters about the perils of drugs, a project that culminated in a brochure on the subject. The project leader said yesterday that Khan had insisted a British flag must be part of the leaflet. "I was born here and I am proud of it," he had said.
The theory that an al-Qa'ida figure known to MI5 may have slipped briefly into Britain to supervise the bombings is becoming increasingly discredited and may be a case of mistaken identity. But Magdi Mahmoud el-Nashar, who was arrested in Cairo on Friday, is still considered important to the investigation.