(Bloomberg) -- T-Mobile US Inc. and the U.S. unit of credit-tracking firm Experian Plc are facing a growing list of lawsuits after hackers gained access to personal data on 15 million T-Mobile customers held on Experian servers.

The hack, revealed Oct. 1, exposed millions of Americans to potential identity theft, forcing T-Mobile customers to take costly actions to protect themselves from fraud, according to complaints filed as soon as a day later. By Wednesday, at least five such suits were under way against both companies. A sixth named only Experian.

Companies and government agencies have been stepping up protection efforts as hackers target troves of personal information that can be sold on the black market and used to carry out financial crimes. Even the most experienced and seemingly protective entities appear vulnerable.

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