The development comes after the Madras High Court handed over the probe to the central agency in November last year rejecting the state CB-CID's plea for more time to crack the case.
Justice A M Basheer Ahamed of the Madurai bench had allowed a plea by the wife of slain leader K N Ramajeyam, Latha Ramajeyam seeking CBI probe into the killing.
The judge was critical of the CB-CID, saying it had been seeking adjournments without any progress in the investigation though five years had elapsed.
He was found gagged and his hands and legs were tied.