Security inside the airport has been stepped up by the Central Industrial Security Force (CISF), while elaborate checks are being conducted outside the premises by Mumbai Police personnel, police said.
The message "ATTACK BY ISIS DATE 10/01/15" had been found written on the wall of a washroom at the Level 2 of Terminal 2 of the Chhatrapati Shivaji International Airport, police said.
"We have been informed about the threatening message. We are looking into the matter. We are taking it very seriously and checking if it was just a mischief," Deputy Police Commissioner (Detection) Dhananjay Kulkarni said.
Although Mumbai police do not have any specific terror warning, they have been maintaining a strict vigil to secure the city against the backdrop of threat from dreaded terror group Islamic State of Iraq and Syria (ISIS) looming large over the country.