An AT&T service outage that affected a huge number of its US customers on Thursday was not caused by a cyber attack, the company’s initial investigation revealed.

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

“Based on our initial review, we believe today’s outage was caused by the implementation and execution of an incorrect process used as we expanded our network, not a cyber attack,” the company he said in the message. “We are continuing 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 was able 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” about the network outage. “If we learn of any malicious activity, we will respond accordingly,” he added.

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

AT&T finally began to change the situation 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 affected customers.

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

Editors’ recommendations