The US provider says the outage was caused by “the application and execution of an incorrect process” during a network expansion.
"We are continuing our assessment of today’s outage to ensure we keep delivering the service that our customers deserve,” AT&T said in a statement.
Over 50,000 incidents were reported by 7am on Thursday and over 70,000 by 9am.
By 10:15am, AT&T said it had restored three-quarters of its network and by 2:10pm, the company restored wireless service to all affected customers.
According to CNN, John Kirby, a national security spokesman, said that the Department of Homeland Security and FBI are looking into the incident and contacting relevant parties.
“The bottom line is we don’t have all the answers to that. I mean, this just happened earlier today. And so we’re working very hard to see if we can get to the ground truth of exactly what happened.”
Capacity will have more on this story as it develops.
