CUDDALORE: Security has been tightened at the Neyveli Lignite Corporation India Limited General Hospital following a bomb threat. A letter on Thursday claimed that a bomb would explode at the hospital between January 31 and February 4. The letter, written in Tamil, claimed that a terrorist from Bengaluru had reached Neyveli to plant the bomb and the act was in protest against the cancellation of the annual medical examination for NLCIL employees.