The breach happened when attackers gained access to a database containing T-Mobile’s information that was run by Experian Plc, the credit-tracking firm, the carrier said Thursday. The hackers stole names, addresses and social security numbers. People who submitted credit applications from Sept. 1, 2013, to Sept. 16, 2015, were affected, T-Mobile said.

“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 US’s chief executive officer, wrote in a letter to consumers.

Experian shares dropped 4.4 percent to 1,028 pence at 8:50 a.m. in London after earlier falling as low as 1,018 pence, the lowest level since December 2014. Deutsche Telekom AG, the parent of T-Mobile US, added 1 percent to 15.21 euros in Frankfurt. T-Mobile US rose 0.8 percent to $40.13 in New York Thursday.

Hackers didn’t gain access to any payment or banking information, Experian said in a statement. The firm is investigating the hack, and said it notified U.S. and international law enforcement. The breach didn’t impact Experian’s consumer-credit database, it said.

Hackers are targeting health care and other companies, after retailers stepped up security following the hack of Target Corp. in 2013, according to John Gunn, a vice president at VASCO Data Security. Hackers can sell social security numbers for 10 to 20 times more than credit-card numbers on the black market, he said.

“My government is not going to send me a new social security number ever,” Gunn said. “There’s exposure for the rest of my life.”