Going full blast against the Congress over the oil-for-food scam, Leader of Opposition Lal Kishenchand Advani on Monday said the agencies investigating the scandal must question Congress president Sonia Gandhi [Images] regarding her party's involvement.

He also said cases must be registered against former external affairs minister Natwar Singh under the Foreign Contribution Regulation Act and Prevention of Corruption Act.

"The Bharatiya Janata Party demands that the Central Bureau of Investigation immediately register a case against Natwar. This alone will enable the CBI and other foreign governments to track the money trail and pinpoint the end beneficiaries in the scam.

"Investigating agencies should also question the Congress president in the course of their probe," he said in Mumbai on the fifth day of his 6000-km long National Integration yatra.

• Coverage: The BJP's twin yatras

The senior BJP leader said, "Natwar is reported to have written at least three letters to Iraqi authorities citing the fraternal links between the Congress and Saddam Hussein's Ba'ath Party to seek benefits under Iraq's food-for-oil programme.

"The involvement of other well-connected Congress leaders has come to light. Thus the taint is spreading despite the best efforts of the Congress and the government to contain it," the former deputy prime minister alleged.

He added: "accountability is an important aspect in democracy. Till date, no explanation has been given by the party [Congress] as to how its name figures in the reports. She [Sonia] has not spoken a word. The only comment from party spokesman is that allegations have no substance."

• Also See: The Volcker scam coverage

Sonia owed an explanation to the country on two counts -- moral and political -- he said.

"Did Natwar Singh write letters to Iraqi authorities without her knowledge and consent?" he asked.

"Why has she still not broken her silence on the Congress too being mentioned by the Volcker Committee as a beneficiary in the scandal? Is it because the Congress, having made Natwar Singh a scapegoat, wants to put the lid on its own involvement in the scandal?" he added.

Advani alleged that the scam was the "latest edition" in the Congress' "long record of taking money from foreign sources".