Experian PLC said a data breach may have compromised the personal information of roughly 15 million consumers in the U.S., including those who had applied for T-Mobile US Inc. subscription services or devices over two years through Sept. 16.

Experian--one of three main credit-reporting agencies along with Equifax Inc. and TransUnion Corp.--said the breach didn't affect the Costa Mesa, Calif., company's consumer-credit database.