K Kamalendran from the Eelam People's Democratic Party (EPDP) and four others were arrested for the murder of the Chairman of the local council of the Jaffna islet of Delft on November 26.
Daniel Rexiyan alias Rajeev was found dead with gun shot wounds at his residence and the police was initially told it was an act of suicide by the 44-year old politician.
CID investigation has shown that the murder was due to an extra marital affair between the northern provincial opposition leader and the dead man's wife, police spokesman Superintendent Ajith Rohana said.
Rexiyan's death led to much speculation that it could have been the work of those who are out to revive the LTTE in the north.
It was seen as the first political murder in the north since the LTTE's defeat four years ago.