The letter addressed to Director R S Sundar and received by post last evening warned that he would be eliminated if he went ahead with commissioning of the plant, police said.
Written in Tamil, the letter signed by "Kudankulam Makkal Nanban" (Friend of Kudankulam people) had been posted from Anna Salai in Chennai, they said.
"The three-tier security arrangement is only for the plant and not for the officials," said the letter received a day after Atomic Energy Commission Chairman Ratan Kumar Sinha said that the first 1000 MW unit of the Indo-Russian project would be commissioned in about two weeks.
"Within this month 100 per cent. It will take about two weeks," Sinha had said about the commissioning of the plant which has already missed several deadlines.
Following the receipt of the letter, a complaint was lodged with Tirunelveli Superintendent of Police Vijayendra S Bidari, who directed police to register a case and investigate.
Meanwhile, Coast Guard and state Marine police organised a joint security drill off Kudankulam coast today.
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The KNPP is already under a thick blanket of security in the wake continuing protests by the local people and fishermen who had attempted to lay a siege to it couple of times as part of their agitation.
The over 500-day long protest against the plant, spearheaded by People's Movement Against Nuclear Energy, citing safety concerns has delayed the commissioning of the plant, first scheduled for December 2011.