Those sources say that Internet chatter and Web postings monitored by authorities are behind the grave concern of an imminent al Qaeda strike, one that at least some law enforcement officials in Britain fear will again target mass transit.
Sir Ian Blair, the head of London's Metropolitan Police, summed up those concerns to the BBC Radio 4 Today program, "The threat of another terrorist attempt is ever present. Christmas is a period when that might happen. We have no specific intelligence to do [with] that.
THE BLOTTER RECOMMENDS
"It is a far graver threat in terms of civilians than either the Cold War or the Second World War. It's a much graver threat than that posed by Irish Republican terrorism," he added.
While there are no specifics to the concern -- no targets, no corroborated intelligence and no very specific timing -- authorities are taking the information more seriously than they otherwise might. Their concern stems from the knowledge that at least 1,200 al Qaeda operatives are at large in Great Britain, with about 18 key al Qaeda members among them and at least two highly placed linchpins to al Qaeda operations, who may have made Britain their home base.
ABC News sources in North America and the U.K. say that British authorities are constantly on the run, breaking up plots on a near weekly basis that have reached the pre-operational phase.
Sources in Britain add that at least one "pivotal...al Qaeda U.K." operative has been detained during the past two weeks, and others may -- at least temporarily -- be in custody.
While authorities have seized some "precursor chemicals" during these raids, including acetone and hydrogen peroxide, as well as batteries, all of these are common household items would serve little purpose in court.
But that is not the purpose of the raids. The purpose is to break up plots before they can be executed.
ABC News has also exclusively learned, with chilling detail, from U.S. and British law enforcement sources that investigators from New Scotland Yard and the British domestic security service MI5 have put together physical evidence and a pattern of interlocking relationships between alleged terrorists that appear to establish a firm link among the subway and bus bombs that killed 52 Londoners on July 7, 2005, a failed set of bombings on July 21, 2005 and a plot to blow up between six and nine airliners, killing as many as 5,000 persons headed to the United States this summer, all the result of three years of planning by British al Qaeda.
Each cell appears to have had ties back to the same British citizen who controlled the plotters from Pakistan and whose identity was first reported by the ABC News Investigative Unit, sources said. That link plus forensic evidence and evidence of overlapping knowledge and personnel in each of the plots is more terrifying to authorities than the prior theory of independent cells operating without knowledge of each others' plans, sources said.
Intelligence sources also say it points to an organized group of cells working to cause carnage and damage to Britain's economy, apparently in an effort to wear down the public will to fight along side the United States in the War on Terror.
Physical evidence includes the use of batteries in more than one plot that cannot be purchased in the United Kingdom but only be purchased in Pakistan, sources say. In addition, a form of the powdered drink Tang most commonly available in Pakistan (although available in some U.K. specialty stores) was the organic material used to mix with peroxide in the bombs intended for the air bombing plot slated for last September to mark the fifth anniversary of 9/11.
Frightening details have also emerged that show how careful the planning was for the latest plot dubbed "Operation Overt," a plot interrupted by authorities at the 11th hour when they felt they had gathered much of the evidence needed to make charges stick.
Sources say the cell members had developed at least four separate methods of potentially delivering their lethal devices: sports drinks, baby bottles, cameras and possibly flashlights. They had also developed four-person teams to board each of the six to nine U.S.-bound commercial jets targeted, with each team member carrying a different apparently innocuous item aboard to maximize chances of avoiding discovery and to maximize the chance of blowing a plane out of the sky by successfully secreting multiple devices in the passenger compartment.
Although the terror suspects were homegrown, the plot itself exhibited none of the hastiness from plan to execution or the lack of professionalism in planning that are often the hallmarks of what are commonly called homegrown plots.
The plot was three years in the making. The chemistry, electronic and concealment skills of the bomb makers in the plot were excellent, intelligence analysts say.
Through it, British authorities came to see that al Qaeda Britain had emerged, and it had emerged as a lethal spin-off of the historical al Qaeda model developed by Osama Bin Laden. Although its ties to that original core of terrorists is largely inspirational, Al Qaeda Britain has the patience, membership and professionalism of its ancestor.
U.S. and U.K. intelligence sources say one of the formidable hurdles British investigators now face is the potential ease with which terrorists and their supporters can conceal their movements within a large, vibrant set of British Muslim communities. There are about two million Muslims in England, about 700,000 in London alone.
And because of a longstanding colonial and post-colonial history with pre-partitioned India and post-partitioned Pakistan, travel between England and Pakistan is substantial. Four hundred thousand British citizens and residents traveled to Pakistan last year, which, sources say, makes identifying a single member from one of 300 cells an enormous challenge.
Alexis Debat contributed to this report