The letter threatening bomb blasts at Sri Ganganagar, Bikaner, Hanumangarh railway stations on March 13, was received by railway authorities in Punjab recently, they said.
"Security has been tightened and SPs of these districts have been directed to remain alert. Directions were also issued to deploy policemen in plain clothes at crowded places," IG Bikaner Govind Gupta said.
Sources said the outfit has also threatened blasts in Punjab, Haryana and Himachal Pradesh.