61.   The FBI is investigating, but has not yet determined the source of the tainted letters.

62.   The individual letters now will be retested to determine the source of contamination.

63.   The jet will be checked to determine the source of the smoke.

64.   The terrorism campaign is harder to define because U.S. authorities are still trying to determine the sources of threats.

65.   The United Nations said it was investigating to determine the source of fire.

66.   U.N. spokesman Alexander Ivanko warned that peacekeepers will shoot back when they can determine the source of the firing.

67.   Veterinary officials were trying to determine the source of the infection, a process that could take several weeks.

68.   Veterinary officials said it could take several weeks to determine the source of the infection.

69.   Veterinary officials were trying to determine the source of the infected pigs, a process that could take several weeks.

70.   Veterinary officials were trying to determine the source, which could take several weeks.