The arrests of two men in Amsterdam for questioning in a terrorism investigation comes at a time U.S. law enforcement officials have been on a heightened state of alert to a possible hijacking of U.S. carrier flights from the Middle East, according to one senior U.S. official. As a result, air marshals were onboard a Chicago-Amsterdam flight yesterday and kept a close watch on two suspicious passengers who had triggered security alarms, but were allowed to travel for "investigative purposes," law enforcement sources told ABC News.
