Mexican authorities arrested a man believed to have killed leading journalist Manuel Buendia five years ago, government officials said. The state-owned news agency Notimex said the alleged assassin, Juan Rafael Moro Avila, 36, belonged to a crack unit of the Federal Security Department. Buendia, whose columns exposed wrongdoing in high places, was shot to death May 30, 1984. Jose Antonio Zorrilla Perez, the former director of the security department, was arrested last week for allegedly ordering the Buendia killing. The Mexico City Attorney General's Office identified the suspect as a member of a prominent family of politicians, and Notimex said he was related to a former president of Mexico.