The Justice Department is looking at Apple's iTunes business, according to the New York Times.
Apple is the largest music retailer in the US, with over a quarter of the market and two-thirds of the online music business.
The inquiry will center around whether the company has used its clout to try and stop music labels from giving exclusive content to Amazon.
It was reported in March that Amazon was asking labels to give it exclusive access to certain songs for their first day of release. In exchange, it offered to include those songs in an 'MP3 Daily Deal' promotion.
But the investigation is likely to have a broader remit, looking at all aspects of Apple's marketing practices.
The probe is in its early stages, says the NYT, with investigators talking to recording labels and other online music retailers. It may not result in a formal inquiry.
Apple's already believed to be the subject of one antitrust investigation. The Justice Department and Federal Trade Commission are said to be looking into Apple's decision to ban developers from using Flash.