CHENNAI: Security was tightened at the Hindu Munnani headquarters in Chintadripet on Saturday after staff there received a threat letter .
In a complaint to police , Hindu Munnani members said the letter warned of dire consequences for TN if the recently arrested suspected ISI operative Sakir Hussain was not released immediately .
Police are now checking to see if it was a prank .
Sakir Hussain was arrested based on a tip from a foreign intelligence agency about a person sent by Pakistan diplomats to spy on possible targets for terror attacks in south India .