British Airways and Air France canceled five flights from Europe to the United States on Saturday after American officials raised concerns about the possibility of an international hijacking.
Continental Airlines also canceled a flight, but it's not clear if the cancellation was related to the others.
Three U.S. intelligence officials told the Washington Post that sketchy intelligence indicates al-Qaida terrorists might want to attempt several new tactics, include releasing an undetectable biological agent such as smallpox or anthrax aboard a plane that passengers would then unknowingly spread; releasing a chemical agent to debilitate the passengers and crew so the plane could be hijacked; and sneaking a radiological device aboard a plane inside a piece of luggage.
The concerns prompted American officials to demand armed sky marshals be placed on those routes. Instead, the airlines canceled the flights.
The groundings revived the trans-Atlantic anxieties seen over the Christmas and New Year's holidays, when more than a dozen flights from England, France and elsewhere were canceled, and travel plans for thousands of international passengers were disrupted, because of hijacking concerns.
British Airways said it was canceling Flight 223 from London to Dulles International Airport in Washington today and Monday and Flight 207 from London to Miami today. The Washington flights were grounded several times after New Year's Day because of specific intelligence pointing to safety risks.
"The British government told British Airways "cancel these flights,' " said John Lampl of the airline.
Air France canceled Flight 26 from Paris to Washington scheduled for today and Monday.
British Airways also canceled its return flights from Washington to London today and Monday because the planes from England would be unavailable. Air France also canceled its return flights.
Continental Airlines Flight 17, scheduled to fly today from Glasgow, Scotland, to Los Angeles with a stop in Newark, N.J., was canceled because the carrier couldn't get the necessary security clearances from the Department of Homeland Security and international counterparts, a Continental spokesman said.
British Airways Flight 223 took off and landed as scheduled Saturday, but as Tampa lawyer Dennis Lopez boarded, he said talk of the cancellations was unnerving.
"I'm a little worried and if I had another flight arrangement right now that could take me there I would definitely take advantage of that," he said.
He said he had just arrived from Kuwait and "breathed a sigh of relief" when he landed in London, thinking he was out of the area of most concern. "It hadn't occurred to me that this flight could be a possible target," he said.
The suspicions concerning a new plot came from a re-examination of the information developed in December and early January and new intelligence developed through informers and other sources, officials said. And the intelligence pointed to an increased risk involving the Air France and British Airways flights to the United States today and Monday, although nothing was on the passenger lists on those flights to arouse concerns, officials said.
American officials have warned that if foreign carriers do not comply with their demand to place armed marshals on flights of concern, they would run the risk of not being allowed into American airspace. The demand has produced resistance in some countries, including England, the United States' main ally in its campaign against terrorism.
Even after the United States lowered the terrorist threat status on Jan. 9, American officials said they were maintaining higher levels of security for airlines and other sensitive sectors.
"Nobody had let down their level of vigilance," Lampl of British Airways said, "and when we have a situation where there is a perceived threat, we will simply cancel the flight."
But he acknowledged this weekend's cancellations "came out of nowhere." He said the flights would be evaluated daily in consultation with the British government.
American officials said they have shared intelligence in recent days with Germany, Spain, Mexico and Canada and alerted them to their renewed concerns about an international terrorist strike.
There is no indication that today's Super Bowl is threatened, but the Federal Aviation Administration imposed flight restrictions over Houston.
_ Information from the Associated Press was used in this report.