Highly-placed sources in the agency said that the Supreme Court has been informed about the probe that was being conducted to ascertain the actual purpose of the land deal.
The CBI has also said that it would be investigating the allocation of dual technology spectrum to Tata Teleservices in February 2008.
The sources said these aspects were not probed during the earlier investigation into the 2G scam. Now these aspects will be looked into thoroughly after which a report will be submitted to the Supreme Court.
Tata Realty spokesperson said the company was "not aware of any preliminary inquiry.
"We have explained our position on this issue in the past. We reiterate that we have comprehensively addressed questions from all Government agencies and fully cooperated with all authorities in their investigations. We stress that we are committed to the highest standards of ethics and business conduct," the spokesperson said.
Unitech refused to comment on the issue.
The agency will probe whether the advance given by Tata Realty was used by Unitech for real estate projects or for a telecom licence pertaining to the 2G telecom spectrum allocation scam.
CBI has already sought a copy of the report of the Serious Fraud Investigation Office (SFIO) which had alleged that the realty deal between Tatas and Unitech in 2007 could have been aimed at funding Unitechs telecom licence.
The need for the probe was felt after Reliance had approached the Supreme Court seeking directions to quash the FIR filed against it in connection with Swan Telecom which was alleged to be the front company of Reliance Telecom Limited.
As the agency was preparing a reply on RTL plea, the CBI found that no probe had been conducted against Tatas so far and the same was conveyed to the Supreme Court.
The CBI would now be examining senior Tata group officials, the sources said.
Besides this, the agency would look into the report of SFIO probe which was ordered by the Ministry of Corporate Affairs in 2012 to look into the affairs of Niira Radia-led PR firm Vaishnavi, which used to handle media relations of various Tata group firms and Unitech, the sources said.
The SFIO report alleged that the Rs 1,700-crore realty deal between Tatas and Unitech in 2007 could have been meant for funding Unitech's telecom licence issued in January 2008.