A special team set up with officials of the STF and detective department was trying to track the number from which the call was made to the national carrier's city office, a senior police officer said.
"After a complaint was lodged with the police on Sunday, STF and detective department have been jointly looking into the case. Security has been tightened in the Air India office," the officer said.
An Air India spokesman said appropriate security agencies like the Bureau of Civil Aviation Security (BCAS) are looking into the issue.
Meanwhile, security has also been upgraded at the NSC Bose International Airport here following the threat call. Security officials at the airport said a Red Alert is already on at the airport in view of the coming Republic Day but additional measures have been taken.
Dog squad has been pressed into service in and around the terminal buildings while each and every luggage was manually checked besides the normal scanning.
The officials said policemen were keeping a tab at the airport's taxi stand and nearby areas.
The call was received at Air India's city booking office at around 5.40 PM and a complaint was subsequently lodged with the Bowbazar police station.