Four transatlantic flights scheduled from Europe to the United States did not take to the skies because of security fears, and two more scheduled for Monday have been canceled.
In Paris, Glasgow, and London, airline check-in staff have been busy trying to shift passengers onto alternative flights. Those affected were travelers on an Air France flight from Paris to Washington, British Airways flights from London to Washington and Miami, and a Continental Airlines flight from Glasgow to Los Angeles via Newark, New Jersey.
All three carriers have chosen to ground the flights, based upon strong recommendations from their respective governments.
A U.S. official said Saturday that the moves were based upon a credible threat from Osama bin Laden's al-Qaida terrorist network.
Meanwhile, The Washington Post, citing information from three intelligence officials, reported Sunday that intelligence information that extremists releasing deadly biological or chemical agents could be on board a plane may be behind the cancellations.
But aviation security expert Chris Yates from the respected Jane's Transport says it remains unclear what exactly is behind the scare that has grounded the passenger jetliners.
"There has been an awful lot of speculation, of course, in the last 24 hours about the actual nature of the threat," he said. "Whether we are talking about a hijacking or whether indeed we are talking about the release of chemical or biological agents on board aircraft. I happen to think that the latter two issues, we could discount."
Mr. Yates believes it has to do with suspicious names that are logged in data bases in the United States.
"We are now into the [Muslim] festival of Eid, and a lot of the people who were traveling out to Mecca, or who traveled out to Mecca are returning over the next few days," he explained. "They are coming through the aviation system. And it remains a strong possibility that their names are being picked up by the authorities and compared with watch lists, and perhaps those matches are actually false."
Meanwhile, the British Airline Pilots' Association has expressed concerns over the strength of the American intelligence reports. The association wants the British government to scrutinize more closely the validity of those threat alerts.
Last month, two British Airways flights were grounded for undisclosed security reasons.