Credit reporting agency Experian on Thursday, Oct. 1, 2015 said that hackers accessed the social security numbers, birthdates and other personal information belonging to about 15 million T-Mobile wireless customers. T-Mobile uses Experian to check the credit of its customers. (AP Photo/Mark Lennihan, File)

(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.

“What makes the most recent breach so ironic is that Experian holds itself out as an expert in the field of data protection, touting its revenues in this area in the amount of $4 billion annually,” one of the plaintiffs said in a complaint.

Experian Information Solutions Inc., the U.S. unit of London-based Experian, held the data on its servers to perform credit checks on current and potential T-Mobile customers. The hackers stole names, addresses and Social Security numbers, T- Mobile said. People who submitted credit applications from Sept. 1, 2013, to Sept. 16, 2015, were affected.

‘Incredibly angry’

“Obviously I am incredibly angry about this data breach and we will institute a thorough review of our relationship with Experian, but right now my top concern and first focus is assisting any and all consumers affected,” John Legere, T- Mobile’s chief executive officer, wrote earlier in a letter to consumers.

California Attorney General Kamala Harris on Monday issued a statement urging the state’s T-Mobile customers and applicants to immediately place fraud alerts on their credit records or pay for security freezes.

The lawsuits, which all seek class-action status, were filed in federal courts in Chicago; Fort Lauderdale, Florida; and Santa Ana, California.

Gerry Tschopp, a spokesman for Experian in the U.S., didn’t immediately return a call seeking comment on the complaints. Timothy O’Regan, a spokesman for Bellevue, Washington-based T- Mobile, a unit of Deutsche Telekom AG, based in Bonn, Germany, didn’t immediately respond to a message.

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