The Kathirur police registered a case under IT Act against Jain Raj, son of CPI(M) district secretary P Jayarajan.
The slain RSS activist E Manoj (42) was the fifth accused in a case of attack on Jayarajan in 1999, from which he narrowly escaped with serious injuries.
Manoj was convicted in the case by the district court but was on bail obtained after filing an appeal in the High Court.
The posting, in Malayalam, was withdrawn after it triggered a controversy with BJP leaders demanding action against Jain. However, within a short span, the post went viral registering several likes and was even shared by many.
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Later, Jain returned with another post recalling how pained he had been when his father was brutally attacked by RSS workers.
Police said the case against Jain was registered under relevant provisions of the IT Act which bars posting comments that would create unrest in society and ill-will among people.