1.   U.S. officials have said the pilots may have become disoriented because fresh heavy snow had covered landmarks and navigational placards.

2.   U.S. officials have speculated the pilots may have become disoriented because the snow may have covered navigational placards on the ground and made landmarks difficult to identify.

3.   U.S. officials say the pilots may have been disoriented by heavy snow that covered navigational placards on the ground and made landmarks difficult to identify.

4.   U.S. officials have speculated that the pilots became disoriented because of heavy snow that covered landmarks and navigational placards.

5.   U.S. officials say the pilots may have been disoriented by heavy snow that covered navigational placards on the ground.

6.   U.S. officials have speculated the pilots became disoriented because a heavy snow may have covered navigational placards on the ground and made landmarks difficult to identify.