A search has also been intensified for the other accused, they said.
A gang led by a history sheeter had attacked the 34-year-old RSS 'karyavahak'and hacked him to death. His left arm was chopped off and there were several injuries all over his body.
City police has extended prohibitory orders for three more days till August 2 and banned processions in the state capital.
The dawn-to-dusk state-wide hartal called by BJP in protest against the incident was almost total.
People who reached railway stations unaware of the hartal had difficulty in getting vehicles to reach their homes.
BJP state president Kummanam Rajasekharan alleged that the CPI(M) was behind the attack.
Describing the murder as brutal, he said this is "annihilation politics" aimed at wiping off the RSS and BJP.
The BJP would take up the matter with the Prime Minister, Home Minister and the National Human Rights Commission, he said.
However, the CPI(M) district leadership has denied any role of the party in the attack.
The incident comes close on the heels of the attack of BJP's Kerala unit office and stone pelting at the house of CPI(M) state secretary Kodiyeri Balakrishnan's son Bineesh Kodiyeri.