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An AT&T service outage that impacted a huge number of its customers across the U.S. on Thursday was not caused by a cyberattack, the company’s initial investigation has revealed.

Instead, the cellular and internet outage was the fault of work that it was carrying out on its systems, AT&T said in a message posted on its website on 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 as we were expanding our network, not a cyberattack,” the company said in the message. “We are continuing our assessment of today’s outage to ensure we keep delivering the service that 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. “We sincerely apologize … Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future,” the company said.

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

The service disruption began in the early hours of Thursday when AT&T customers started reporting difficulties placing calls, sending texts, and accessing the internet. Early on, the Down Detector site showed more than 32,000 reported outages across AT&T’s network. As more people began their day, the number climbed, with 75,000 outages reported by around 9:15 a.m. ET.

AT&T finally began to turn the situation around later in the 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 our affected customers.”

While it’s reassuring to know that the outage doesn’t appear to have been caused by malicious activities, customers will be eager for more details about the incident, as well as information on what action AT&T is planning to take to ensure that it doesn’t happen again.

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