Key intelligence that led to security officials fearing a terrorist cell was to launch a large-scale bomb attack on British soil came from MI6 operations targeting Pakistan, the Guardian has learned.
Sources with knowledge of the investigation say the intelligence gathered first from abroad and then in Britain before Wednesday's raids, left counterterrorism officials believing there was too high a risk of an "imminent" attempt to bomb targets in Britain and cause mass casualties.
However, subsequent, intensive searches of at least 10 addresses linked to the suspected al-Qaida plot in northern Britain have so far failed to turn up any clear evidence of a terrorist conspiracy, despite the huge resources devoted.
Forty-eight hours after the arrests, sources close to the inquiry say no evidence has been found of bombs, bomb-making parts, precursor chemicals to make explosives, a bomb factory, weapons or ammunition.
The jigsaw of intelligence studied before the Wednesday raids left all agencies – from the police, to security services to Whitehall officials – convinced they had to arrest the 12 people detained. Today warrants were granted for the continued detention for a further seven days of 11 of the men, while an 18-year-old man was released into the custody of the UK Borders Agency.
A Greater Manchester Police spokeswoman said: "The North West Counter Terrorism Unit was last night given warrants for the further detention of 11 men arrested." Police now have a further seven days to detain the men, who range in age from 22 to 41 and are in custody in various locations across the country.
The 18-year-old has been released without charge in relation to the anti-terror operation and is now in the custody of the UK Borders Agency, which regulates immigration and can investigate the status of those entering the country.
Police are continuing to search 10 addresses in Liverpool and the Cheetham Hill area of Greater Manchester.
Examination of computers recovered from raids in Manchester, Liverpool and Clitheroe has only just begun. In past cases, data recovered from computers has been a source of incriminating evidence. Other forensic tests are also at an early stage.
One senior source said "nothing of huge significance" had been found, and accepted it was possible that no evidence could emerge before the 28-day limit to hold terrorism suspect expires.
But the source added: "There is lots of material that when put together may take us somewhere. It will be a long and drawn out investigation.
"If we don't charge it will look bad, but we had to take action. Was there any alternative; no there wasn't."
A central mystery remains how counterterrorism officials could believe such a serious plot existed when they were unsure of seemingly basic elements of the alleged conspiracy, such as the targets.
The Guardian has spoken to several informed sources with knowledge of the case, who paint a picture of fragments of intelligence coming in and fuelling a mounting concern that Britain was facing one of it's worst terrorist atrocities. They spoke on condition of anonymity.
Two to three weeks ago, intelligence from operations carried out in Pakistan by MI6, Britain's foreign intelligence agency, provided names of suspected members of an al-Qaida trained network in Britain.
That intelligence as well as the interception of calls and emails led UK security officials and police to fear a large terrorist attack was being planned in Britain.
The monitoring of calls and emails also led officials to believe a link existed between suspected al-Qaida commanders in Pakistan and the people in the UK.
An informed source said the plot was "al-Qaida driven". "There was concern [al-Qaida] was directing operations ... recruiting people to travel to this country, direct their operations and tell them what to do."
A large surveillance operation was put into place.
Some of those arrested were seen taking photographs near a Manchester nightclub, the Birdcage, and the Trafford shopping centre. The suspects were also seen taking photographs near other places in and around Manchester which are usually crowded.
The source said the suspect's behaviour was consistent with the way terrorists carry out reconnaissance ahead of an attack. But it was also consistent with a group of students in a new area taking photographs of themselves to show friends and family.