The United States had put other nations on notice earlier this week that it would not allow certain suspicious flights into its air space without armed marshals on board.
Seven international flights have now been canceled since last Saturday after the Bush administration began an aggressive approach to defending its air space when the nation was put on orange or "high" alert on Dec. 21. Administration officials said no arrests had been made in connection with any of the more than a dozen international flights subjected to rigorous scrutiny. And officials have acknowledged that even now, they are uncertain whether they have succeeded in foiling a terrorist plot.
First they say that the only reason the flights were cancelled was that the pilots refused to fly with air marshalls on board.
The very next paragraph states that they're uncertain if they have succeeded in foiling a terrorist plot.