It quickly emerged that two of the bombers, Mohammad Sidique Khan and Shehzad Tanweer, had spent several months in Pakistan.
They told families and friends they had gone to undertake religious studies, but it is thought they may have undergone training or made contact with extremists.
The pair flew together from London to Karachi via Istanbul on 19 November 2004 and were photographed at the airport as part of Pakistan's Pisces security system (see right).
After arriving in Pakistan, they appear to have split up, with Tanweer visiting his uncle's house in Faisalabad. They regrouped a week or so later and possibly went to the Afghanistan border area.
The official investigation says it is likely that they had some contact with al-Qaeda figures and may have received training.
They left the country three months later on 8 February 2005.
Before this visit, Khan is thought to have had some training in a remote part of Pakistan during a two week trip in 2003.
Hasib Hussain had also visited Pakistan at some point with his family, as had Tanweer.
After the London bombings, Pakistan police raided Islamic schools, or madrassas, across the country.