Fox News reports AT&T said the network disruption that caused tens of thousands of customers to lose cellular service yesterday was not the result of a cyberattack.

After service was fully restored at around 6 p.m. Eastern, AT&T revealed the results of its initial investigation into the outage.

“Based on our initial review, we believe the outage was caused by the application & execution of an incorrect process used as we were expanding our network, not a cyberattack. We are continuing our assessment to ensure we keep delivering the service that our customers deserve.”

AT&T’s service suffered a national outage yesterday, beginning at about 3:30 a.m. Eastern. Just before 2 p.m. Eastern, the number of reports had declined drastically to nearly 4,900 after spiking to more than 73,000 just after 9 a.m. Eastern.

Downdetector, a website that tracks phone and internet service outages, reported AT&T service was down mostly in the Atlanta, Chicago, Houston, and Miami service areas.

A U.S. defense officials told Fox News there was no indication it was a cyber-attack.

The FBI and Federal Communications Commission (FCC) said they were aware of the outages and investigating.

“We are aware of the reported wireless outages, and our Public Safety and Homeland Security Bureau is actively investigating,” the FCC posted on X. “We are in touch with AT&T and public safety authorities, including FirstNet, as well as other providers.”

AT&T apologized for the outage yesterday after several irate customers complained about being unable to make calls or send texts on social media.

Downdetector showed a spike in reported outages for T-Mobile and Verizon customers as well yesterday, but both companies said their networks were operating normally and suggested customers were having trouble attempting to connect to users on other networks.