This is unusual, to say the least. Is it also improper? At least one expert thinks so.

The judge who presided over the biggest terrorism trial since Sept. 11 left his Detroit courtroom, traveled to CIA headquarters and helped interview a witness whose testimony later became key to the judge's reversal of convictions in the case.

Government officials familiar with the interview told The Associated Press the judge and Justice officials worked together outside the presence of defense lawyers to conduct the interview because of concerns about protecting secret information under the Classified Information Procedures Act. But legal experts said U.S. District Judge Gerald Rosen's actions were highly unusual and could provide grounds for lawyers to challenge his impartiality because he assumed the role of investigator in a case he continued to preside over.