T-Mobile US Inc said personal data of about 15 million of its U.S. applicants may have been stolen in a breach at a unit of Experian Plc, which processes the telecom carrier’s credit applications.

The records include names, dates of birth, addresses and encrypted fields with Social Security number and ID number, as well as additional information used in T-Mobile’s own credit assessment.

T-Mobile had 58.9 million customers in total as of June 30.

Payment card or banking information were not acquired, T-Mobile Chief Executive John Legere said in a letter on Thursday.

Experian determined that the encryption may have been compromised, T-Mobile said.

The data pertains to applicants requiring a credit check for service or device financing from Sept. 1, 2013, through Sept. 16, 2015, Experian North America said in a statement.

Federal and international law enforcement agencies were notified and an investigation was ongoing, Experian North America said.

Experian is the world’s biggest consumer credit checking agency and is best known for running consumer credit checks for banks, landlords and retailers.

T-Mobile’s shares were down 1.7 percent in extended trading after closing little changed at $40.13 on the New York Stock Exchange. (Reporting By Arathy S Nair in Bengaluru; Editing by Sriraj Kalluvila)