AT&T’s outage that affected a large number of its customers across the United States on Thursday was not caused by a cyberattack, the company’s initial investigation revealed.

Instead, the cell and Internet outage was the fault of work it was doing on its systems, AT&T said in a message posted to its website Thursday evening.

“Based on our initial review, we believe that today’s outage was caused by the application and execution of an incorrect process used while we were expanding our network, not by a cyber attack,” the company said in the message. “We continue our assessment of today’s outage to ensure we continue to provide the service our customers deserve.”

Earlier in the day, at 3:10 p.m., AT&T said it had managed to restore wireless service to all of its affected customers, adding: “We sincerely apologize… Keeping our customers connected remains our top priority, and “We are taking steps to ensure our customers will not experience this again in the future.”

The FBI said in a widely reported statement that it was “in contact with AT&T” regarding the network outage, adding: “If we become aware of any malicious activity, we will respond accordingly.”

The service outage began in the early hours of Thursday when AT&T customers began reporting difficulties making calls, sending text messages and accessing the Internet. Initially, the Down Detector site showed more than 32,000 reported outages on AT&T’s network. As more people started their day, the number increased, with 75,000 outages reported around 9:15 a.m. ET.

AT&T finally began to turn things around later that morning, reporting at 11:15 a.m. ET that it had restored “three-quarters” of its network. Then, at 3:10 p.m. ET, the company confirmed that it had “restored wireless service to all of our affected customers.”

While it’s reassuring to know that the outage does not appear to have been caused by malicious activity, customers will be interested in more details about the incident, as well as information about what steps AT&T plans to take to ensure it doesn’t. It happens again.

By Sam