A 42 year old former LTTE cadre was detained by Sri Lanka's Terrorism Investigation Division on Wednesday in Jaffna.
The man, Markandu Nevinathan, left the LTTE in 1997 and then went on to live in Kodikaamam, where he got married and had four children.
The TID officiers arrived from Colombo and detained him, before transferring him to Colombo's TID "fourth floor" office, notorious for its use of torture.
His relatives say they do not why he has been arrested.
Mr Nevinathan left the Vanni region with his family following the end of the armed conflict and tried to make a living fishing in the Thenmaratchchi-Kachchai region.