Source: Fear of bioweapons grounded flights
But intelligence information said to be spotty, unreliable
West Virginia Sen. Jay Rockefeller says a biological or chemical attack on an airliner could not be stopped.
CNN's Elaine Quijano reports that fears of a biological attack were possibly behind flight cancellations.
British Airways and Air France face renewed terror threats against U.S.-bound flights.
Despite the cancellation of international flights, travelers seem determined not to give in to security fears
WASHINGTON (CNN) -- The specter of a biological attack was raised Sunday as a possible reason that a handful of transatlantic and domestic flights were canceled this weekend.
One federal law enforcement source told CNN that the U.S.-bound flights were grounded mainly out of fears that terrorists would use the planes as "air taxis" to deliver biological, chemical or radioactive weapons material to cities in the United States.
The source said the intelligence information was spotty and may be unreliable, and added that translation problems made the picture even less clear.
According to this source, the intelligence centered on British Airways, Air France and several U.S.-owned airlines, which the source did not identify. In some cases, specific flights were highlighted, some of them through several weeks in the future.
All flights mentioned in that intelligence have been canceled, a government official said.
The federal law enforcement source also said that at least one federal agency, the Department of Energy, was concerned enough about the latest intelligence that it asked that cities install radioactivity detectors.
The developments come as U.S. officials, citing credible electronic intercepts, say al Qaeda may again be targeting international flights into the United States.
A senior administration official said the intelligence gives precise threat information, including airlines, dates and flight numbers.
The intelligence mentioned Air France and British Airways flights to the United States, as well as British Airways Flight 223 specifically.
A key member of the Senate Intelligence Committee said the United States would have no way to counter a biological or chemical attack on a U.S.-bound airliner.
"I don't think so, and that's partly the problem of not checking cargo, and it's partly the problem of biological weapons, which nobody has figured out really what to do about yet," West Virginia Sen. Jay Rockefeller, a Democrat, said on "Fox News Sunday." "Nobody has any idea about what to do about them on an airplane or on the ground."
Outside the network's studios, Rockefeller added, "We don't know how to protect against any biological. ... You play it safe, and the plane doesn't fly, and people are going to have to get used to that, and people are not going to like that, but it's what you've got to do in this era."
British Airways, Air France and Continental Airlines grounded several flights to the United States for security reasons this weekend. (Full story)
Three Air France flights to Los Angeles, California, were canceled Christmas Eve and Christmas Day because of similar threats.
British Airways canceled Flight 223 from London to Washington Dulles Airport on Sunday and Monday, and the return flight, Flight 222, both days. Flight 207 from London to Miami, Florida, on Sunday was called off as well.
"We canceled these flights on advice from the U.K. government for security reasons," a spokeswoman for the airline said.
Air France canceled Flight 026 from Paris to Washington on Sunday and Monday, the airline said.
Also, Flight 378 from Paris to Philadelphia, Pennsylvania, was grounded Saturday, but the Air France Web site said it was called off for "operational reasons."
Continental Airlines also canceled Flight 1519 from Washington to Houston, Texas, on Sunday because of security concerns, spokesman David Messing said.
"We weren't able to obtain the necessary security clearance from the Department of Homeland Security," Messing said.
He said the cancellation was not caused by the Super Bowl, being played in Houston.
A senior U.S. official said the airlines, not the U.S. government, decided to cancel the flights.
"We did not want to cancel" the French and British flights, the official said. "We have been working all week to try and prevent that. Once it gets into the airlines' hands, however, then this is what happens."