The letter was received at the office of the Director General of Police here and the Chennai Press Club yesterday.
A stencilled letter in red ink claiming to be from the "ISIS" said that "identified 50 RSS active terrorists are in Chennai. We swear to shoot at least 10 of them and repeat Kovai blasts by April 24 - ISIS."
On February 14, 1998 serial bomb blasts shook Coimbatore in Tamil Nadu in which 58 were killed and over 200 injured.
A senior police official said the letter, its origin and other relevant aspects are being probed.