An alleged terror plot to blow up several transatlantic passenger jets, which British police said they had thwarted on Thursday, would have caused even greater casualties than 9/11, officials and experts say, marking a chilling departure from smaller-scale terror attacks of recent years.
Thousands were stranded at Britain's biggest airports Thursday and delays rippled across the US as flights were canceled and check-in security ramped up to surreal proportions after police said they had arrested 21 suspects in connection with a plot to smuggle liquid explosives onto several aircraft.
Officials in London and Washington said the alleged plotters, many believed to be British-born, had their sights on United, American, and Continental flights. Simultaneous explosions were to have been caused by chemicals in carry-on luggage. Officials monitored the plot for months before deciding they had to move.
Experts say that the foiled attack suggests Al Qaeda involvement. Aviation is still a favored target for Al Qaeda acolytes bent on taking terrorism to new heights.
"The best way to top the 9/11 event is to do it with civilian aircraft with a lot of people on board," says Rolf Tophoven, a German terrorism expert. "This will create huge damage on all kinds of economic and commercial levels."
Bruce Hoffman, an expert on terrorism at the RAND Corp. in Washington, notes that it's typical of Al Qaeda to go back to targets and improve their techniques on past attacks. The successful attack on the USS Cole in 2000 followed a failed bid to sink the USS Sullivan in 1999. The 9/11 attack came eight years after the limited attack on the World Trade Center in 1993.
This latest effort, he adds, is a carbon copy of the failed 1995 "Bojinka" plot by Ramzi Ahmed Yousef, convicted of being the mastermind of the 1993 World Trade Center attack, to blow up 11 American airliners over the Pacific, using plastic explosives.
Britain raised its terror alert levels to the highest notch, while the US put a red alert – the first time that level was invoked – on flights from Britain. All other US flights were one step below, at "orange."
US Homeland Security Secretary Michael Chertoff and Peter Clarke, deputy assistnant commissioner for Scotland Yard, said the plot was international in scope and involved many people.
President Bush, in Wisconsin, said that the US was safer than it was before 9/11. "We've taken a lot of measures to protect the American people but obviously we're not completely safe," he said. But, he added, "It is a mistake to believe there is no threat to the United States of America."
Officials would not say exactly how many aircraft were targeted, nor when precisely they thought the plotters would strike. French officials said Thursday that the plotters were probably of Pakistani origin. British officials refused comment.
The number of suspects involved points to a far bigger operation than the post-9/11 terror attacks in London; Madrid; Bali; and Casablanca, Morocco. Paul Wilkinson, a security expert and author of "Terrorism Versus Democracy," says: "It's the scale of conspiracy that is so worrying because it's much larger than most things we've seen since 9/11."
Yet attacking aviation targets has proved harder since 9/11 because of heightened security. To subvert that vigilance, the plotters arrested Thursday were allegedly planning to smuggle aboard different liquid chemicals that may appear innocuous but could be explosive if mixed.
As a result, passengers with babies Thursday were ordered to take a swig of their baby formula in front of security guards to demonstrate its integrity. Personal documents, medication, and all other hand luggage was banned from flights out of Britain.
The unravelling of the plot marks another apparent success for British counterterrorism, which already claims to have foiled a dozen or so attacks since 9/11, four in the past year. While some of these have produced no convictions and inconclusive evidence, security expert Bob Ayers says this one should be taken seriously.
"You don't take a decision to shut down all flights to America lightly," says Mr. Ayers, an expert with the Chatham House think tank. "It would suggest they have substantial evidence."
He adds that British intelligence must have somehow infiltrated the plotters – a major success for organizations that are scrambling to get up to speed with the new terror threat from radical, disenchanted segments of the Muslim community. "There could be more than one cell involved here, but this means [the intelligence services] have penetrated more than one group or the supporting infrastructure."
Security officials had a delicate task knowing when to move, said Home Secretary John Reid said. "Move too early, you may not know the full scope of who is involved and you may provoke those you don't know into taking the action you don't want. If you don't move, you run the risk of terrible consequences."
But while the "underresourced and underappreciated" security services have done commendable work, says M.J. Gohel, a terrorism analyst at London's Asia Pacific Foundation, they've had less success finding out who is providing "homegrown" terrorists with direction.
British officials regularly warn the public of the threat. Wednesday, Mr. Reid said that the country faced its most sustained period of threat since World War II.
Britain is a particular target because of its backing for US foreign policy in the Middle East. RAND's Hoffman also notes that the fifth anniversary of 9/11 is looming. Despite comments that Al Qaeda may be in retreat, he says, "nothing is further from the truth. At least in Europe, Al Qaeda has put down an organizational structure to sustain these attacks."
• Correspondents Christa Case and Faye Bowers contributed to this report.