The judge in the first civilian trial of a Guantanamo Bay detainee barred the prosecution's star witness yesterday from testifying, dealing a major setback to the Government's effort to build criminal cases with evidence obtained through harsh CIA interrogations overseas.

United States District Judge Lewis A. Kaplan said the witness could not take the stand because investigators learned of his existence through coercive questioning of the defendant, terrorism suspect Ahmed Khalfan Ghailani, at a secret CIA camp.