The Enforcement Directorate (ED) on Friday registered preliminary enquiry in the National Herald case to find out if there was any money laundering angle involved in the case.
The enquiry was set up on the complaint of BJP leader Subramanian Swamy. The ED director entrusted the headquarter investigation unit with the task.
The action was taken on a day when the Delhi High Court would hear Congress plea against the income tax summons to party chief Sonia Gandhi and vice-president Rahul Gandhi.
Congress leaders Kapil Sibal and Abhishek Manu Singhvi would defend the party in the court.
Earlier, Congress president Sonia Gandhi had accused the government of vindictiveness in the wake of an income tax notice in the National Herald case, asserting that such "political witch hunting will only help us come back".
A Delhi court had ordered process on a private criminal complaint filed by BJP leader Subramanian Swamy against Sonia Gandhi, Rahul Gandhi and others in a new acquiring the assets of a company that ran the now-defunct National Herald newspaper.