Nigil, Biju, Riyaz, Vineesh Vazhakkadan, P Sumesh and P P Basheer were involved in a six year-old case relating to the murder of Sainudheen (a political worker) and they were also fined Rs 50,000 each by the court.
CBI took over the probe in the case from the Kerala police in 2008 on the directions of the Kerala High Court.
"It was alleged that the place Kakkayangad situated in the southern part of Kannur district had a strong influence of a leading political party. A new political organisation was started in 2003 for the uplift of a particular community and was growing very rapidly in the district of Kannur which clashed with the interest of other existing political parties over the control of that community.
Sainudheen was attacked by five men on June 23, 2008 and he later died in a local hospital the same day.
The slain workers' mother had approached the High Court and filed a writ petition after which the court handed over the probe to the central agency.