It began with tales of a "terror alert in US and Britain" and culminated in a "plot to bomb Heathrow". In the past few days anyone following the American and British media or listening to US security chiefs would be in little doubt the two countries were facing an imminent attack by al-Qa'ida.
New information is, however, emerging that calls into doubt the level of threat. Senior British counter-terrorist sources yesterday denied they had found any specific plot to attack Heathrow or any other British airport.
The source of the alarming stories about a plan to bomb Britain's biggest airport appears to be four-year-old information held by an al-Qa'ida computer expert arrested in Pakistan last month. Mohammed Naeem Noor Khan, who was arrested on 13 July, had stored photographs of Heathrow, as well as pictures of underpasses beneath several buildings in London, Pakistani intelligence officials say.
But these images are understood to have been non-specific, several years old, and did not include details or plans of an attack. They are considered to be general intelligence material held by the terrorist network, which is known to favour targeting airport and aircraft, as well as financial institutions.
Mr Khan was under investigation for several months by MI5 before his arrest and it was his links with a suspected network of al-Qa'ida supporters, including an alleged high-ranking commander in Britain, that led to the raids in London and the home counties on Tuesday. Mr Khan, 25, has visited Britain on several occasions and some of his UK contacts have been the subject of MI5 surveillance.
The 12 men of Asian origin who were arrested during the operation are continuing to be held in London, where they are being questioned by anti-terrorist detectives. Ten of the arrested men, who are all aged in their 20s and 30s, are UK citizens.
All are being detained on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism after raids on addresses in London, Bushey in Hertfordshire, Luton, and Blackburn. Police did not find any bomb- making equipment or chemicals in the raids.
Among those arrested is reported to be the alleged senior al-Qa'ida member, known as Abu Eisa al-Hindi or Abu Musa al-Hindi - although no one of that name is being held in London.
Scotland Yard's anti-terrorist branch was forced to bring forward plans to arrest the suspected al-Qa'ida cell after news of Mr Khan's arrest leaked out on Monday.
News of the latest threat emerged on Sunday night when intelligence officials in Washington briefed US journalists on what they said was a "treasure trove" of new information indicating that al-Qa'ida was targeting financial institutions in three east coast cities.
It was quickly reported that the information had come from the unannounced arrest of two al-Qa'ida suspects in Pakistan in July: much of it was contained on a laptop computer seized on 25 July following a 12-hour gun fight that ended with the capture of Ahmed Khalfan Ghailani, wanted for his alleged role in the 1998 bombing of two American embassies in Africa.
US officials said that the CIA had tipped off the Pakistani authorities about the location of Mr Ghailani's safehouse in Gujrat after they tracked down Mr Khan, who had e-mailed the information to Mr Ghailani 12 days earlier.
But on Monday, with authorities closing streets in New York and around the Capitol in Washington and armed officers boarding Metro trains, causing traffic chaos and public anxiety - officials were forced into retreat. The surveillance information, it was admitted, was up to four years old and pre-dated the attacks of 2001.
To confuse matters further, officials now insist there is new intelligence - some of it based on information provided by an al-Qa'ida suspect in British custody. Reports in the US media say it was this information that persuaded officials to raise the terror alert on Sunday evening.
Meanwhile, there was a fresh warning about other possible terrorist targets. Sir Alan West, Britain's First Sea Lord and Chief of the Naval Staff, said yesterday that intelligence showed that merchant navy vessels were regarded as prime targets for attacks. "They've realised how important it is for world trade in general; they understand the significance," Sir Alan said.