T-Mobile has alerted its U.S. wireless customers to a recent hack that exposed some of their personal information.

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.”