Flags of the NATO nations at half mast 12 September 2001 Please credit NATO photos. (NATO Photo 414Kb) Flags of the 19 member countries flying at half mast at the main entrance to NATO Headquarters in Brussels following the terrorist attack on the United States of 11 September (NATO Photo 508Kb) Flags of the 19 member countries flying at half mast at the main entrance to NATO Headquarters in Brussels following the terrorist attack on the United States of 11 September (NATO Photo 411Kb) 12 September 2001 Security check at the main entrance to NATO Headquarters following the terrorist attack on the United States of 11 September. Flags of the 19 member countries flying at half mast in the background (NATO Photo 533Kb) Flags of the 19 member countries flying at half mast at the main entrance to NATO Headquarters in Brussels following the terrorist attack on the United States of 11 September