The letter was received at the LIC headquarters on May 17 in which the sender claimed to be from SIMI and said that the building would be blown up on May 24 if their arrested activists in Indore and Lucknow were not released.
"The LIC authorities had given us the letter on Saturday, following which security at the building has been increased," Deputy Commissioner of Police (Zone I) Brijesh Singh said.
In light of the current scenario, the letter had to be taken seriously and additional security presence was deployed at the building, he added.