A brother of one of the 24 suspects seized by detectives investigating a plot to bomb up to 12 planes was seized in Pakistan shortly before police launched their raids, officials said on Friday.
The arrest of Rashid Rauf in the border area with Afghanistan was a trigger that led anti-terrorist investigators to start an immediate pre-emptive operation with officers fearing that the alleged cells were ready to strike.
Pakistani officials claimed on Friday night that Rauf had links with al-Qaeda.
"We arrested him from the border area and on his disclosure we shared the information with British authorities, which led to further arrests in Britain," said Pakistani Interior Minister Aftab Khan Sherpao.
Foreign Minister Khursheed Kasuri said that Rauf had been monitored for some time before his arrest. Rauf's uncle was murdered in Birmingham in April 2002 and as part of the murder hunt it is understood that Rauf's home in St Margaret's Road in the city was searched.
The arrest of Rauf in Pakistan was one of seven made by Pakistani authorities in recent days, and is understood to have included one other Briton. Rauf's two brothers were arrested in Birmingham on Thursday as part of the raids.
It also emerged on Friday that at least one of the suspects arrested in the district of Walthamstow, east London, regularly attended Islamic camps run by Tablighi Jamaat, an organization which the US believes has been used as a recruiting ground for al-Qaeda.
Martyrdom tapes and other significant items were discovered during the search of the 29 properties where arrests were made on Thursday morning.
As it became clear that Pakistan played a pivotal role in the plot, and helped in its unmasking, British counter-terrorism officials said several of the Britons involved had visited the country two months ago, before returning to the UK.
British intelligence sources say the original tip-off about the alleged plot came more than a year ago from an informant in the UK. The informant is believed to have come from the Muslim community.
A combination of Rauf's arrest, at least one intercepted message from Pakistan to Britain, and an alert from an informant here, led to Thursday's arrests, according to British security sources.
More details about the backgrounds of the 24 arrested suspects emerged on Friday. Three were Muslim converts. The youngest was 17 and the oldest 35. Among those arrested was the wife of one of the suspects, detained with her young baby.
It is likely that in the next few days a handful of those arrested will be released, having been caught up in the sweep inadvertently. It is understood that the 19 names released by the Bank of England as subject to asset freezes are the core suspects.
Although some had visited Pakistan, a senior security official said: "The plot was constructed in the UK, targeted in the UK, based in the UK, and foiled in the UK."
But it is not clear when the attack was to take place. None of the alleged plotters had yet bought airline tickets, according to anti-terrorist sources.
US and British counter-terrorism officials believe that the liquid chemical the alleged plotters were planning to use to crash the planes was the peroxide based TATP.