An independent data protection authority has slapped SK Telecom(SKT) with a record-high penalty of nearly 134-point-eight billion won, or around 97-point-one million U.S. dollars, over a data breach that resulted from a hacking incident in April.

According to the Personal Information Protection Commission on Thursday, the fine was decided at a plenary session the previous day after the commission concluded that the nation’s leading mobile carrier neglected safety measures and failed to protect its subscribers’ personal data.

The commission imposed an additional penalty of nine-point-six million won because SKT did not notify the affected subscribers within 72 hours of learning of the data breach.

The investigators found that the leak compromised 25 types of data, including mobile phone numbers for the carrier’s 23 million users and SIM verification keys, after multiple systems at the core of SKT’s mobile service were hacked.

The commission said the carrier had operated and managed its systems despite security vulnerabilities between the internet and its internal networks, and was careless in its efforts to detect and respond to hacking attempts, even violating some of its own regulations.