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 cyber attack," 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."