Records show a third bomber, Hasib Hussain, 18, who blew up the No 30 bus, arrived in Karachi last July on a Saudi Arabian Airlines flight SV-714. His date of departure is not known.
MI5 is investigating reports from the Pakistan security services that the three men from Leeds went to several madrassas - religious schools, some of which are run by terrorist groups.
Mounting evidence points towards Pakistan as the likely source of inspiration and training for the bombers, although the identity of the chief planner and bomb-maker is unknown.
Pakistani intelligence officials have said Tanweer stayed briefly at a madrassa and met a member of an outlawed domestic militant group. Pakistani intelligence agents have questioned students, teachers and administrators at the school in Lahore, and at least two other al-Qa'ida-linked radical Islamic centres.
Pervez Musharraf, the President of Pakistan, said that some Islamic schools were involved in terrorism. His comments came after an intelligence official named one of five militants detained by security forces at the weekend as Qari Usman, a bomb expert from the banned militant organisations Jaish-e-Mohammed (Army of Mohammed). The militants were detained in Faisalabad as part of a crackdown after the London bombings, although no link has been established.
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, two US intelligence officials said. Khan was investigated by MI5 last year but was not deemed to be a terrorist threat.
He was subject to a routine assessment by MI5 because of an indirect connection to an alleged plot to build a lorry bomb in Britain. He was one of hundreds investigated but not considered a risk.
Police are still trying to identify the type of homemade explosive used.
Ministry of Defence experts are examining chemicals and other bomb-making equipment found at a house in Leeds, where the bombs are thought to have been made, and a hire car used by the three Leeds bombers.
Initially, forensic specialists thought the explosive used was a home-made substance, nicknamed "the mother of Satan". The explosive, acetone peroxide, can be produced by following instructions on the internet. But there is now doubt about the nature of the material discovered. The explosives were carried in rucksacks bought from Blacks camping shop in Leeds city centre.
Explosive experts are also continuing to examine two cars used by the bombers and left at Luton train station on the morning of the attacks. The bombers travelled from Luton on the train, before dispersing and setting off the explosives.
A senior security source said: "It will take some time for the forensic investigation to show what explosives were used and in what way."
No completed bombs have been discovered by the police and they have not found any evidence of a second suicide unit.
Forensic analysis has also failed to find timers at the blast sites, suggesting that the bombers detonated the explosives by hand.
Scotland Yard's anti-terrorist branch is still trying to establish the exact movements of the bombers leading up to the London attacks. Officers are examining all possible routes from three of the bombers' homes in Leeds and those for the Jamaican-born Germaine Lindsay, 19, from his home in Aylesbury, Buckinghamshire. They have seized more than 12,000 surveillance tapes, many from motorway service stations, and plan to examine a further 13,000.Reuse content