The investigation is being led by the US Justice Department's environment and natural resources division, the source told AFP, speaking on condition of anonymity and confirming media reports.
The German automaker is already under investigation by the US Environmental Protection Agency, and could face up to USD 18 billion in fines related to that inquiry.
The California Air Resources Board is also investigating Volkswagen's violations of emissions standards.
Today, Volkswagen admitted that 11 million of its diesel cars worldwide are equipped with devices that can cheat pollution tests.
A hearing over the emissions scandal will be held soon in Congress, two Republican lawmakers in the House of Representatives announced yesterday.