Nagapattinam: A bomb threat has been issued to Periya Palli Vasal (mosque) near Seerkazhiin in the Nagapattinam district of Tamil Nadu, following which security has been beefed up, police said on Saturday. An anonymous letter saying the bomb would go off at the place of worship before this month-end was received by the police. A case has been registered a case and investigation has begun, the police said.