AT&T says three-quarters of its network has been restored after a mobile phone outage disrupted calls and text messages for thousands of US users, prompting an investigation by federal agencies.
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The wireless carrier, whose 5G network covers about 290 million people across the United States, has been grappling with interruptions to its service for more than nine hours.
The Federal Communications Commission said it was investigating the incident while the US Cybersecurity and Infrastructure Security Agency (CISA) said it was working with AT&T to understand the cause.
White House spokesman John Kirby said the FBI and the Department of Homeland Security (DHS) were looking into the AT&T outage.
CISA is a unit of DHS.
Kirby added that the FCC in touch with the company.
"But the bottom line is we don't have all the answers," he said about the cause of the outage.
The outage affected people's ability to reach emergency services by dialling 911, according to posts on the X social media platform by government departments in several US cities.
Doug Madory, the director of internet analysis firm Kentik, said it was unclear from the outside what exactly had happened at AT&T, although he said he doubted it was the result of malicious activity.
"I'm sceptical this was some kind of an attack," he said in a telephone interview.
"Most of these things end up being some kind of a software push that got screwed up somehow."
AT&T shares were down 2.2 per cent in afternoon trading.
There were about 4300 incidents of outages reported at 1.51pm ET on Thursday, according to tracking website Downdetector.com, down from more than 70,000 earlier in the day.
"We are working as quickly as possible to restore service to remaining customers," AT&T said on its website.
It did not provide a reason for the outage or the number of affected users.
"We are aware of an issue impacting AT&T wireless customers from making and receiving any phone calls (including to 911), the San Francisco Fire Department said on X.
The Prince William County Police Department in Virginia and Charlotte-Mecklenburg Police Department in North Carolina posted similar statements on the platform.
Users of Verizon, T-Mobile and UScellular also faced disruptions but they were more limited than the AT&T reports, Downdetector showed.
The other companies said their network was operating normally and the outage was potentially related to customers trying to connect with other networks.
US Senator Rick Scott of Florida said in a post on X that he had contacted AT&T for an update.
"Florida law enforcement is doing everything it can to keep people safe, and I expect AT&T to keep us informed on what it is doing to get 911 services fully back online ASAP," he said.
Some iPhone users have seen SOS messages displayed in the status bar on their mobile phones.
The message indicates that the device is having trouble connecting to their phone provider's network but it can make emergency calls through other carrier networks, according to Apple Support.
In 2021, regulators settled for $US19.5 million ($A29.6 million) a probe into a T-Mobile outage during the pandemic that lasted more than 12 hours and led to more than 20,000 failed 911 emergency calls.
with AP
Australian Associated Press
