The letters written purportedly by 'Makkal Podhunala Kuzhu' (Public Welfare Committee of the People) warned that bombs would explode inside the plant complex on May 21.
The KNPP authorities handed over the letters to the police.
Meanwhile, leaders of People's Movement Against Nuclear Energy, spearheading the stir against the project, said it has no role in the threat letters.
S P Udayakumar, Convener of PMANE, said they have been leading a peaceful, democratic and non-violent struggle for the past nine months without even a single incident of violence.
