In a letter posted to its website, the telecommunications company revealed that personal information, including customer names, phone numbers, e-mail addresses, and account numbers, were recently exposed to hackers. The carrier said that it discovered the breach on Monday, closed the hole that allowed hackers in, and informed authorities. It’s unclear, however, whether the exposed data was actually stolen.

“We take the security of your information very seriously and have a number of safeguards in place to protect your personal information from unauthorized access,” T-Mobile wrote on its website. “We truly regret that this incident occurred and are so sorry for any inconvenience this has caused you.”

T-Mobile didn’t say in the letter how many of its 77 million customers were affected. The carrier revealed to Motherboard that 3% of its customers were affected, or about 2 million to 2.5 million people.

Of course, hacking and corporate security breaches have become all too commonplace. In many cases, hackers obtain records to sell on the so-called Dark Web, a corner of the Internet where illegal goods are readily available for purchase. In other cases, hackers simply breach companies to make a point about corporate security and embarrass their targets.

T-Mobile’s letter didn’t say what the hackers’ motivation might have been.

According to T-Mobile, it has notified its affected customers, so those who do not receive a notification are not believed to have been involved in the breach. Some of its MetroPCS customers were also affected by the breach and were similarly notified.

T-Mobile declined to comment further in response to Fortune inquiry.