AT&T says the outage to its US cellphone network was not caused by a cyberattack

AT&T said the hours-long outage on its US mobile network Thursday did not appear to be the product of a malicious assault.

The outage disrupted mobile service for thousands of customers across the United States beginning early Thursday before being restored.

“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 cyber attack,” the Dallas-based company said.

The outage tracker Downdetector observed that the disruptions peaked at around 73,000 reported instances, having started at roughly 3:30 a.m. ET. Around 12:00 ET, AT&T had more than 58,000 outages in cities including Houston, Atlanta, and Chicago. It is the biggest carrier in the nation, with over 240 million customers.

Less than 1,000 reports had been made on AT&T’s network by 8 p.m. ET.

AT&T-owned Cricket Wireless experienced over 9,000 outages at one time, but later in the afternoon, reports of the outages also decreased. Other carriers, such as Verizon and T-Mobile, acknowledged receiving complaints from consumers about problems, but claimed their networks were up and running and that the problems were probably caused by customers attempting to connect to AT&T subscribers.

During the outage, some iPhone users saw SOS messages displayed in the status bar on their cellphones. The message indicates that the device is having trouble connecting to their cellular provider’s network, but it can make emergency calls through other carrier networks, according to Apple Support.

The Federal Communications Commission contacted AT&T about the outage and the Department of Homeland Security and FBI were also looking into it, National Security Council spokesman John Kirby said.

The FBI acknowledged it had been in touch with AT&T. “Should we learn of any malicious activity we will respond accordingly,” the agency said.

The outage also raised concerns on Capitol Hill.

“We are working to assess today’s disruption in order to gain a complete understanding of what went wrong and what can be done to prevent future incidents like this from occurring,” said a statement issued by Cathy McMorris Rodgers, a Washington Republican who chairs the House Energy and Commerce Committee, and Ohio Republican Bob Latta, chair of the Communications and Technology Subcommittee.