T-Mobile blamed a bum fiber-optic circuit for Monday’s massive network outage that rankled customers and prompted a federal investigation.

Neville Ray, the wireless carrier’s president of technology, issued a statement Tuesday explaining the cause of the failure that crippled certain voice calls and text messages for roughly 13 hours.

T-Mobile engineers traced the outage to a “fiber circuit failure” linked to a third-party provider in the Southeast, Ray said. The backup system for the circuit also failed, leading to a surge in traffic that spread from the Southeast and overwhelmed network supporting the affected voice calls, he said.

“I want to personally apologize for any inconvenience that we created yesterday and thank you for your patience as we worked through the situation toward resolution,” Ray said in the statement.

Ray did not name the provider responsible for the circuit that failed, but he said T-Mobile is working to add “permanent additional safeguards” to prevent future outages like Monday’s. The company is also trying to figure out what caused the initial overload failure, he added.

Ray’s statement came after Federal Communications Commission chairman Ajit Pai said his agency would investigate the outage amid frustration from T-Mobile customers, some of whom took to Twitter to demand discounts or credits to make up for the interruption.

T-Mobile in April completed its $26 billion merger with Sprint, which a federal court approved despite concerns that it would hurt competition and increase prices for consumers. Customers on Sprint’s network were not affected by Monday’s outage, during which data connections continued to function, Ray said.