A former Salvadoran army intelligence agent who said he participated in ``death squad'' operations in his homeland was sentenced to six months in prison Friday for illegally entering the United States. Cesar Vielman Joya-Martinez was sentenced by U.S. District Judge Claude M. Hilton. Joya-Martinez, convicted Sept. 19, could have received a two-year prison term. The 27-year-old Joya-Martinez has said he took part in death squad operations of the Salvadoran army's First Brigade. Joya-Martinez, who had been deported from the United States in 1983, was arrested last spring after he appeared at an administrative hearing to determine if he were eligible for political asylum. Joya-Martinez's attorney, Daniel S. Alcorn, said his client returned to the United States because he feared Salvadoran soldiers were plotting to kill him to make him a scapegoat for death-squad killings.