S17, E19: A Russian emigre is poisoned and what appears to be an act of terrorism turns into a revenge plot -- Karl Rostov (guest star Michael Aronov) dies at the hospital after symptoms of a severe stomach flu. The FBI are under the impression that they are dealing with terrorism until the medical examiner finds Ricin in Rostov's body. As Detectives Ed Green (Jesse L. Martin) and Nina Cassady (Milena Govich) dig further a mystery surrounding Rostov's life is unraveled. S. Epatha Merkerson, Fred Dalton Thompson and Alana De La Garza also star.