The Justice Department and the FTC are both interested in examining whether Apple is running afoul of U.S. antitrust laws by funneling media companies' customers into the payment system for its iTunes store—and taking a 30% cut, the people familiar with the situation said. The agencies both enforce federal antitrust laws and would have to decide which one of them would take the lead in the matter.

Representatives of the Justice Department, the FTC and Apple all declined to comment.

Apple's rules don't stop media companies from selling digital subscriptions on their own. But the company imposed restrictions that could make that option less attractive to customers, and steer more sales through its own system.