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%. 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 on Saturday.
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.