Eight of them were members of the gang that hacked to death Marxist rebel Chandrasekharan while three others were CPI(M) activists who were found to have plotted the murder.
One of the 12 accused found guilty by the Special Judge R Narayayana Pisharadi was awarded three years' imprisonment.
The verdict came as an indictment for the CPI(M) as the judge observed that the motive of the crime was political and not personal enmity.
The party activists sentenced to life imprisonment were P K Kunhanandan, a committee member from Panur in Kannur district, K C Ramachandran, a local committee secretary and Manoj, a branch secretary.
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Chandrasekharan, a former CPI(M) wholetimer from the red bastion Onjiyam near Vatakara in north Kerala, was hacked to death by an eight-member gang on May 4, 2012.
The killer gang had came by a hired car and brutally hacked to death the target by inflicting 51 cuts on his body.
The murder had cast a shadow on the CPI(M) in state firmly controlled by state secretary Pinarayi Vijayan with not only rival parties but senior leaders like V S Achuthanandan using it as a weapon to attack internal foes.
Reacting to the sentence, Chandrasekharan's widow K K Rema said it came as relief to a great extent as it laid bare the CPI(M)'s involvement in the conspiracy to annihilate her husband.