International health experts probing Nigeria's first recorded death from the bird flu virus have not been able to determine the source of the infection, a senior official of the Food and Agriculture Organization said Saturday. All the team found was that the chicken which caused the infection that killed a 22-year-old woman on Jan. 17 was purchased from a live bird market in Nigeria's biggest city of Lagos just before Christmas, said Tony Forman, leader of the U.N. team of experts helping Nigeria in the probe. It was not clear how the chicken contracted bird flu.