The chief witness against a paratrooper accused of murder during the U.S. action in Panama last year acknowledged that he shot an unarmed Panamanian in the back. 1st Lt. Brandon Thomas, testifying in the court-martial of Master Sgt. Roberto Bryan at Ft. Bragg, N.C., said he had taken that Panamanian prisoner and was searching him when he ran. He said he did not know the man was unarmed. Bryan is accused of shooting another Panamanian without provocation Dec. 23, three days after the United States invaded Panama to oust Manuel A. Noriega.