The case had been heard before a special jury for the first time in history and it is the first-ever occasion that a verdict was given after midnight.
Nadaraja Raviraj, who represented the Tamil National Alliance (TNA) from the northern Jaffna district, was shot dead outside his home in his car in Colombo in November, 2006.
The police's Crime Investigations Department had found the vehicle and the weapons allegedly used for the murder.
The suspects held were all Naval intelligence personnel who were acquitted and released after they were found not guilty by a special jury. The decision was reached after midnight following a trial which lasted over a month.
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One of the suspects had died during the trial, according to reports.
The jury has decided that the accused cannot be convicted based only on the fact that they were identified by several witnesses in the case.
The High Court tried three of the suspects in absentia as they were absconding since the beginning of the nvestigations.
Another suspect former Police Constable Manamperige Sanjaya Preethi Viraj, had turned state witness in the case.
He was released Thursday after in the remand custody for almost 20 months.
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