Three of the four London suicide bombers visited Pakistan last year, officials there have confirmed.
Mohammad Sidique Khan pictured at Karachi airport in November
Mohammad Sidique Khan and Shehzad Tanweer arrived and left together, and spent three months in the country.
Hasib Hussain flew in last July. It is not clear how long he stayed. Security officials have been trying to establish what the men did during their visits.
The three, all Britons of Pakistani descent, and one other bomber were among 56 people killed in the blasts.
Police have confirmed they were the UK's first suicide bombings.
The three men, all from the Leeds area of northern England, were tracked by a system called Pisces in which everyone who comes into Pakistan legally, via any port of entry, is photographed.
Mohammad Sidique Khan, 30, and Shehzad Tanweer, 22, flew into Karachi together on Turkish Airlines flight 1056 on 19 November, 2004 and both left on Turkish Airlines flight 1057 on 8 February this year.
Records show Hasib Hussain, 18, arriving in Karachi last July, on Saudi Arabian Airlines flight SV-714. His port of exit has not been established.
What the men did during their visits is also not clear.
There has been no official confirmation of reports that Shehzad Tanweer visited both the eastern cities of Lahore and Faisalabad.
His family says he attended an Islamic school, or madrassa, during his most recent visit.
Pakistani security officials have previously said he visited the country briefly on at least one other occasion, possibly at the end of 2003.
Intelligence officials want to know whether the men were in contact with the al-Qaeda network or other Islamic militant groups operating in Pakistan, or if there was a Pakistan-based mastermind behind the attacks on 7 July.
Shehzad Tanweer (above): Aged 22, born Bradford, lived Beeston, Leeds. Studied religion in Pakistan. Forensic evidence linking him to Aldgate blast.
Mohammad Sidique Khan: Aged 30, from Beeston, Leeds, recently moved to Dewsbury, married with baby. ID found at Edgware Road blast site.
Hasib Mir Hussain: Aged 18, lived Holbeck, Leeds. Reported missing on day of bombings. Said to have turned very religious two years ago. ID found in No 30 bus.
Germaine Lindsay: Jamaican-born man living in Buckinghamshire. Believed to have carried out King's Cross attack.
Sources say Pakistani intelligence and investigation agencies are working flat out to accommodate British demands for leads on any of the three London bombers of Pakistani descent.
On Sunday in an attempt to find out more about the bombers' final movements, UK police released a CCTV image of them as they set out from Luton.
They also confirmed the names of all four men for the first time. The fourth was a Jamaican-born Briton, Germaine Lindsay, 19.
Three bombs exploded on the London underground at 0850 BST, and one on a bus at 0947.
It is thought Hussain was responsible for the bus bombing, in which 13 people died, Khan the Edgware Road blast that killed six people; Tanweer for the Aldgate blast, which killed six, and Lindsay for the Russell Square explosion where 26 people were killed.
More than 700 other people were injured in the explosions.
On Monday, Pakistani President Pervez Musharraf condemned the attacks.
He told a youth convention in Islamabad: "Terrorism in the name of Islam, launching bomb attacks in London in the name of Islam, is not Islam.
"The entire nation has to reject extremism."