One of the men who carried out last week's serial bombings in London had visited Pakistan twice and spent a total of four months there, officials said.
However, they have so far not been able to pinpoint the movements of the bomber Shehzad Tanweer in the country or say who he met.
According to his uncle in Islamabad, Tanweer had been to Pakistan and attended a madrassa or Islamic school.
Quoting knowledgeable sources, BBC said 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.
The sources said Tanweer first visited Pakistan possibly at the end of 2003, then for a longer period later.
His entries were tracked by a system called Pisces in which everyone who comes into Pakistan legally, via any port of entry, is photographed.
Agencies in Pakistan are still trying to trace Tanweer's movements inside the country. The other two bombers of Pakistani descent had not been picked up on the Pisces system. So if they did enter Pakistan after 2002, when the system was introduced, they did so illegally.
The sources said little was known about the bombers in Pakistan, but more may be known about the people they may have been in contact with.
That, and efforts to ascertain whether there was a Pakistan-based mastermind, will be the focus of investigations in the coming days and weeks, the report said.