Congress President Sonia Gandhi with her son and party vice president Rahul Gandhi (File Photo)
The Enforcement Directorate has filed a preliminary investigation into allegations of cheating and criminal breach of trust in a case filed by BJP leader Subramanian Swamy.
Sources say the inquiry has been registered "just to establish whether there is a case or not."
Congress leaders have alleged "political vindictiveness" after the new BJP government took power in May.
Earlier this week, Sonia and Rahul Gandhi challenged an order summoning them to a Delhi court on August 7 to respond to Mr Swamy's allegations.
Mr Swamy claims that the Gandhis broke the law to grab valuable properties in Delhi, including the office of the National Herald, a newspaper that was set up before independence by Jawaharlal Nehru. It was closed down in 2008 by Mrs Gandhi, whose husband Rajiv was Mr Nehru's grandson.
In June, metropolitan magistrate Gomati Manocha, who is hearing the case said, "I have found prima facie evidence against all the accused. The court has directed them to appear before it on August 7."
The Gandhis and the Congress have denied the allegations. "It is sub-judice," Sonia Gandhi said today, asked to comment on the enforcement directorate's inquiry.
Political parties are exempt from taxes on all sources of income. But Mr Swamy alleges the Congress broke the law by loaning Rs. 90 crore to a firm called Young Indian where the majority stake is owned by the Gandhis. Political parties cannot give loans for commercial transactions.
Mr Swamy alleges that Young Indian acquired Associated Journals Limited, the publisher of three newspapers, including the National Herald, and that the Gandhis aimed to grab property worth thousands of crores that belonged to the publishing company.
Congress, however, pointed out that Young Indian is a not-for-profit company and its directors are not even allowed any remuneration.