A threatening letter demanding the release of suspected Indian Mujahideen terrorists arrested from the state over the past few days was today delivered to the Kota SP's office.

"The threatening letter came to my office by ordinary post in which the sender demanded release of the terrorists arrested in the state in the last few days," Kota SP Satveer Singh told PTI.

The letter threatened of consequences if the terrorists arrested from Jaipur, Jodhpur and Sikar were not released, following which security arrangements were tightened, he said. Singh said in the letter the sender's name was mentioned as "Rafiq".

The SP said senior officials at the police headquarters and the ATS had been communicated about the letter.

Investigation is on to identify and arrest the sender, he added. Five suspected IM terrorists were recently arrested from Rajasthan. Zia Ur Rehman alias Waqas, a Pakistani national wanted in connection with several bombings, was arrested outside Ajmer Railway Station on March 22.

At his instance, his three accomplices--Mohammad Mahruf (21), Mohammad Waqar Azhar alias Haneef (21), both residents of Jaipur, and Shaquib Ansari alias Khalid (25) of Jodhpur were arrested from their residences the next day.

Later, Addas (22) was arrested from Sikar on March 30.