Judicial sources said Abdul Qadir Abdul Qadir, who rents a plot of land near where Tueni was killed, was being questioned about phone calls he made just before and after the Dec. 12 explosion.
Tueni and three other people were killed in a car bomb attack as he was traveling through the Christian-dominated area of Beirut. It was so powerful it blew his armored car off the mountain road and sent it rolling down the hill.
Last month, the Lebanese government asked the U.N. Security Council to investigate Tueni's killing and other recent political murders, including that former Prime Minister Rafik Hariri in February.
Damascus has repeatedly denied any involvement in the killings.