The letter, delivered by an unidentified person to Ambani's personal staff in a sealed envelope at his Maker Chambers office on February 24, also said IM would damage his 27-storey mansion 'Antilia' on Altamount Road, a police officer said on condition of anonymity.
The handwritten letter, claimed to have been penned by an IM operative, said the organisation wanted the release of its operative Danish and, if he was not set free, the home-grown terror group would cause harm to Ambani.
Police suspect the letter referred to Mohammed Danish Ansari, a native of Darbhanga in Bihar, who was arrested in January by NIA for sheltering Bhatkal brothers before they reportedly escaped to Pakistan three years back.
After receiving the letter, a senior officer of RIL met city Police Commissioner Satyapal Singh to apprise him of the development, the officer said.
Police would ask for CCTV footage from the security staff of Ambani's office to find out whether any images of the person, who had delivered the letter, were captured.
When contacted, a spokesman for RIL refused to comment as the matter was under investigation.
RIL shares eyed.