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

The hack, revealed Oct. 1, exposed millions of Americans topotential identity theft, forcing T-Mobile customers to take costlyactions to protect themselves from fraud, according to complaintsfiled as soon as a day later. By Wednesday, at least five suchsuits were under way against both companies. A sixth named onlyExperian.

Companies and government agencies have been stepping upprotection efforts as hackers target troves of personal informationthat can be sold on the black market and used to carry outfinancial crimes. Even the most experienced and seeminglyprotective entities appear vulnerable.

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

Experian Information Solutions Inc., the U.S. unit ofLondon-based Experian, held the data on its servers to performcredit checks on current and potential T-Mobile customers. Thehackers 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 thoroughreview of our relationship with Experian, but right now my topconcern and first focus is assisting any and all consumersaffected,” John Legere, T- Mobile’s chief executive officer, wroteearlier in a letter to consumers.

California Attorney General Kamala Harris on Monday issued astatement urging the state’s T-Mobile customers and applicants toimmediately place fraud alerts on their credit records or pay forsecurity freezes.

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

Gerry Tschopp, a spokesman for Experian in the U.S., didn’timmediately 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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