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Wireless provider T-Mobile USA Inc. (NASDAQ: TMUS) is notifying between 2 million and 3 million customers that some of their personal information was stolen by hackers on Monday, August 20. A company spokesperson told tech website Motherboard that around 3% of the company’s 77 million users were affected.

The company said it discovered the attack shortly after it was launched and shut it down quickly. Personal information that may have been exposed included name, billing zip code, phone number, email address, account number, and account type (pre- or post-paid). No financial information such as credit card numbers or Social Security numbers were exposed, nor were any user passwords compromised.

T-Mobile’s spokesperson told Motherboard that the hackers were part of “an international group” that gained access to customer data that “didn’t contain any financial data or other very sensitive data.” The spokesperson declined to give specific information on the attack and did not know whether the attackers were criminals or part of a government-backed group.

The company tried to reassure customers that those affected would be notified by T-Mobile:

All affected customers have been, or shortly will be, notified. If you don’t receive a notification that means your account was not among those impacted by this incident. If you need assistance or have questions about this incident or your account, please contact Customer Care at your convenience.

T-Mobile’s notification includes further details for customers on how to get in touch with the company.