"We will trace the source the funds of the companies including the 18 companies that invested in Purti," a top IT official told PTI on condition of anonymity.
He said prima facie 'shell' companies (inactive firms used as a vehicle for various financial manoeuvres or kept dormant for future use in some other capacity) had pumped money into PPSL.
Sources in the department said a report would be submitted to the Central Board of Direct Taxes shortly.
"We have already started gathering documents relating to these companies," they said, adding officials in the investigation wing of the department in Mumbai and Pune were involved in the probe.
Officials of the investing companies would be summoned in due course and, if needed, the BJP chief would also be asked to be present, the sources said.
Gadkari is at the centre of a raging controversy over funding of PPSL with media reports claiming that major investments and large loans to Purti were made by Ideal Road Builders (IRB) group, which had won contracts between 1995 and 1999, when the BJP chief was PWD minister in Maharashtra.
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Media reports also claimed that shell companies were floated for making investments and their fake addresses were submitted to the Registrar of Companies.
On Tuesday, Corporate Affairs Minister Veerappa Moily had said that Registrars of Companies would inquire into the allegations.
On his part, Gadkari has denied the allegations and said he was prepared for a probe.