Tom Ridge observing new security measure at airport

It has become an agonizing daily judgment call: whether to cancel international flights on the basis of fragmentary intelligence warning of a September 11-style attack.

But that is what has happened several times since the U.S. terror alert status was raised last month. Flights from London to Washington and from Paris and Mexico City to Los Angeles have been sporadically grounded, delayed or tailed by U.S. fighter jets.

Western intelligence officials say they know just enough to believe that specific flights are at risk, but that they do not have enough information to be able to track down and arrest anyone.

Security analysts say the information is probably coming from intercepts of e-mails or phone calls that are only yielding scraps of data, such as a specific flight number.

The French interior ministry said last week that several Air France Paris to Los Angeles flights were grounded in December after U.S. investigators misidentified six passengers as suspected terrorists. French justice authorities say they are looking for a passenger who failed to show up for a December 24 Air France flight to Los Angeles that was eventually canceled for security reasons.

Analyst, Philip Baum, editor of Aviation Security International, says it is difficult to establish reliable watch lists of potential terrorists.

"One part of the equation is that they are looking at passenger lists, looking at people's names that are appearing on lists," he said. "But that is throwing up a lot of false positives, people that are being identified as potential risks, but turning out to be perfectly safe passengers."

But Mr. Baum says British Airways twice suspended its flight 223 to Washington last week, probably because of specific intelligence rather than concerns about names on the passenger list.

"At the end of the day, we know that we can screen passengers at airports and we can deny them boarding if we want to," he said. "I think the information out there would indicate that there is a possibility of an attack on operating certain routes, be they from France, from the United Kingdom, from Mexico. That is what they are worried about. That is the focus."

Mr. Baum says the other possible reason for the suspension of the British Airways flights was that the pilots decided that, if the threat was so high, they would rather not fly than obey Washington's orders to put armed marshals on the flight.

British pilots have been among the most recalcitrant about allowing armed guards on their flights. Jim McAuslan, the head of the British Air Line Pilots Association, explains why.

"Once you try and deal with one risk, the risk of terrorism, you create another risk, which is the risk of safety, of having arms in aircraft," he said. "But if we accept that they are going to be there, I think our second concern is how they are deployed, about the quality of the police sky marshals, about the sort of weapons they carry, and, ultimately, about who is in command of the aircraft."

Britain's Transport Secretary, Alastair Darling, defends the deployment of marshals as responsible and prudent and says they will be used where appropriate.

"There is an increased threat, and we have to deal with that in a balanced and proportionate way," he said. "Our objective is to ensure that we deploy all the security measures available to us, as and when appropriate, while at the same time enabling people to go about their day-to-day business."

Mr. Darling struck a deal Tuesday with the pilots' union whereby passengers will not know when an air marshal is aboard, though the pilot will know and will remain in charge of the aircraft. But Mr. Mc Auslan says a formal agreement is still needed.

"Until we have an agreed protocol in place, the advice to our own members who are confronted with a police sky marshal is you do not fly," he said.

Some other European countries do not have such qualms.

Germany began to use air marshals on flights to the United States and elsewhere soon after the September 11 attacks. Switzerland has deployed them randomly on international flights for more than 30 years. And France, which shares the tough U.S. approach to terrorism, put marshals on its civilian aircraft in the past few weeks.

Portugal, Sweden, and Denmark say they will refuse to conduct flights carrying armed guards. And Europe's biggest charter carrier, the German-owned Thomas Cook Airlines, has vowed to cancel any flight rather than carry sky marshals.

Some airline analysts say the presence of sky marshals is likely to raise fears among passengers. But Simon Evans, of Britain's Air Transport Users' Council - a passenger advocacy group - says travelers will see the marshals as a step toward increased security.

"I think, overall, people probably would feel very sure that there is a presence there, somebody trained who knows what they are doing when a terrorist - hopefully it does not happen - but if the terrorist gets on board the plane and starts threatening people," he said. Another issue is the cost of placing marshals on aircraft. Who is going to pay for them? The International Air Transport Association, which represents the world's major airlines, says governments should foot the bill for enhanced security. Andrew Brookes, an aerospace analyst at the International Institute for Strategic Studies in London, says governments have little choice but to do so.

"There are a lot of airlines out there that are on the brink of bankruptcy anyway," he said. "They are only kept going by governments, so, at the end of the day, will the government pick up the tab for Air this, Air that, Air the other? It probably will. It probably won't give compensation to passengers necessarily. It will keep the airline flying for prestige purposes."

Most international airlines have left little doubt that, although they do not like the new U.S. measures, they realize that they have little choice but to go along with them or face being shut out of the lucrative trans-Atlantic market.