In a legal filing with the US International Trade Commission, Apple has revealed that the US Department of Justice is investigating Samsung over its alleged use of FRAND patents to harm competition. An investigation was hinted at back in June in a Bloomberg report, and Dow Jones reports today that the DOJ has been working on the probe for several months. The agency's interest in investigating FRAND-related matters, however, is not a surprise: after approving Motorola's merger with Google earlier this year, the DOJ said that it will be on guard against improper use of essential patents in the mobile industry and "will not hesitate to take appropriate enforcement action" against violators.

Apple's filing with the ITC is the latest in its ongoing dispute with Samsung over the company's use of its 3G patents: Apple (and others) have raised concern with Samsung's use of standards essential patents to harm competitors. Samsung was targeted back in January by an EU antitrust investigation over the company's unfair use of FRAND patents against competitors. Earlier this year, Samsung lost its cast against Apple, when a jury ruled that none of the iPhone manufacturer's devices infringe Samsung's 3G networking patents. The Department of Justice hasn't confirmed the investigation yet, and it's not clear what "appropriate enforcement action" it might use against FRAND violators, but Samsung may have more hurdles ahead if it wants to keep lobbying 3G patent claims at competitors.