CPI-M state unit Secretary G Ramakrishnan, who led the protesters numbering around 700, said a CBI inquiry alone would bring out the truth behind the suicide of the official.
He alleged that the present CB-CID investigation was only a farce and would not bring out the truth as claimed by the government.
Muthukumarasamy, an Executive Engineer in the agriculture department, killed himself by jumping in front of a train at Thatchanallur in the district on February 20.
Following the protests, then Agriculture Minister 'Agri' S S Krishnamoorthy was dropped from the Cabinet and also stripped of his post in AIADMK as Tiruvannamalai south district secretary.
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Ramakrishnan said in the last 10 months, as many as 100 murders had taken place in the southern districts, mostly due to caste factor or sand mafia, but the police were indifferent.
Though the government claimed the law and order was good, these murders showed that the situation was out of control.
Based on call records and statements by agriculture officials, the police team is probing the role of the former minister's personal assistants.