The no-fly list created by U.S. authorities, which singles out passengers who are potential terrorist threats, is the target of frequent criticism that it's incomplete and unreliable. But that hasn't stopped it from expanding dramatically. Aviation sources say the list has grown to more than 31,000, up from 19,000 last September. And a little noticed incident on April 8, involving a Dutch KLM 747 flight from Amsterdam to Mexico City, may result in the list being used even more aggressively. The plane was forbidden by American authorities to enter U.S. airspace because the Department of Homeland Security discovered after the flight had taken off that two of its passengers were on the no-fly list. According to government sources, the two were Saudi men who had undergone pilot training with Sept. 11 hijacker Hani Hanjour. The flight was turned back and landed in London, where the men were questioned by Dutch authorities and allowed to go because they were not on any Dutch watch list. Now, in the wake of the KLM incident, the Transportation Security Administration is seeking to expand the use of the no-fly list, proposing that all foreign airlines—even those not flying to a U.S. destination—check their manifests against the list if they are flying over U.S. airspace. That has already raised hackles. Some airline experts say it may contravene international agreements and could cause major disruptions in the coming summer travel season. "This could open up the U.S. to retaliation," says a former transportation official. Overflight rights are long established in international skies, he notes, and restricting them "would be much more of a burden for U.S. airlines, which fly over many more countries than foreign airlines passing through U.S. airspace."